Hydrodeoxygenation of bio-derived
oxygenates over bifunctional metal-acid
catalysts in the gas phase
Thesis submitted in accordance with the requirements of the
University of Liverpool for the degree of Doctor in Philosophy
by
Khadijah Hamed Alharbi
July 2017
i
Abstract
Hydrodeoxygenation of bio-derived oxygenates over
bifunctional metal-acid catalysts in the gas phase
PhD thesis by Khadijah Alharbi
Biomass-derived organic oxygenates, such as ketones, carboxylic acids, alcohols, phenols, ethers
and esters, obtained from fermentation, acid-catalysed hydrolysis and fast pyrolysis of biomass
are attractive as renewable raw materials for the production of value-added chemicals and bio-
fuels. For fuel applications, these oxygenates require reduction in oxygen content to increase
their caloric value. Much current research is focused on the deoxygenation of renewable organic
oxygenates using heterogeneous catalysis.
The aim of this thesis is to investigate the hydrodeoxygenation (HDO) of organic oxygenates,
including aliphatic and aromatic ketones, ethers and esters, using bifunctional metal-acid
catalysis in the gas phase to produce value-added chemicals and bio-fuels. The bifunctional
catalysts comprise Pt, Ru, Ni and Cu as the metal components and a caesium acidic salt of
Keggin-type tungstophosphoric heteropoly acid (H3PW12O40, HPA), Cs2.5H0.5PW12O40 (CsPW),
as the acid component, with the main focus on the Pt–CsPW catalyst. We also report enhancing
effect of Au on the HDO of ketones over Pt-CsPW. A variety of techniques were used to
characterise these catalysts. These techniques include BET, TGA, gas chemisorption, ammonia
adsorption calorimetry, STEM-EDX, XRD, ICP, elemental analysis (C, H combustion analysis)
and FTIR.
ii
The bifunctional catalysed HDO of ketones to form alkanes occurs via a sequence of steps
involving hydrogenation of ketone to alcohol on metal sites followed by dehydration of alcohol
to alkene on acid sites and finally hydrogenation of alkene to alkane on metal sites.
It is demonstrated that the bifunctional HDO pathway is more efficient than the monofunctional
metal catalysis. Catalyst activity decreases in the order of metals: Pt > Ru >> Ni > Cu.
0.5%Pt/CsPW is a versatile catalyst for the HDO of aliphatic ketones, giving almost 100% alkane
yield at 100 oC and 1 bar pressure. Evidence is provided that the reaction with Pt/CsPW at 100
oC is limited by ketone-to-alcohol hydrogenation, whereas at lower temperatures (≤ 60 oC) by
alcohol dehydration resulting in formation of alcohol as the main product. The catalyst composed
of a physical mixture Pt/C + CsPW is highly efficient as well, which indicates that the reaction
is not limited by migration of intermediates between metal and acid sites in the bifunctional
catalyst. Notably, the mixed Pt/C + CsPW shows better performance stability in acetophenone
HDO as compared to the Pt/CsPW catalyst, which suffers from deactivation.
We also demonstrate enhancing effect of gold on the activity and stability of Pt/CsPW catalyst
in HDO of 3-pentanone. Gold additives increase the turnover rate of 3-pentanone HDO at Pt
sites. In addition, the bimetallic catalyst PtAu/CsPW shows the preference of C=O over C=C
bond hydrogenation in comparison to the unmodified Pt/CsPW catalyst. STEM-EDX and XRD
analysis indicates the presence of bimetallic nanoparticles with a wide range of Pt/Au atomic
ratios in the PtAu/CsPW catalysts. The gold enhancing effect on the HDO of ketones over Pt-
CsPW can be attributed to PtAu alloy effects (ensemble and ligand effects). Catalyst modification
with gold can be a promising methodology to enhance the HDO of biomass-derived feedstock
using platinum group metal catalysts.
iii
In the HDO of ethers and esters, including the aromatic ether anisole, the aliphatic diisopropyl
ether (DPE) and the aliphatic ester ethyl propanoate (EP), bifunctional metal-acid catalysis is
also more efficient in comparison to the corresponding monofunctional metal and acid catalysis.
Moreover, it has been found that metal- and acid-catalysed pathways play a different role in these
reactions.
Hydrodeoxygenation of anisole is a model for the deoxygenation of lignin. With Pt-CsPW, it
occurs with almost 100% yield of cyclohexane under very mild conditions at 60-100 oC and 1
bar H2 pressure. In this reaction, Pt-catalysed hydrogenation plays the key role, with a relatively
moderate assistance of acid catalysis, further increasing the cyclohexane selectivity. The
preferred catalyst formulation is a uniform physical mixture of Pt/C or Pt/SiO2 with excess
CsPW, with a Pt content of 0.1-0.5%, which provides much higher activity and better catalyst
stability to deactivation as compared to the Pt/CsPW catalyst prepared by impregnation of
platinum onto CsPW. The Pt/C + CsPW mixed catalyst has the highest activity in anisole
deoxygenation for a gas-phase catalyst system reported so far. In contrast to anisole, the aliphatic
ether DPE decomposes readily over CsPW via acid-catalysed pathway (E1 mechanism) without
metal assistance to give propene and isopropanol. Propene selectivity increases with reaction
temperature at the expense of isopropanol. Platinum alone (Pt/C), in the absence of CsPW, is
inactive in this reaction, either under H2 or N2. However, in the presence of Pt-CsPW under H2,
DPE decomposition is significantly accelerated, yielding the more thermodynamically favorable
product propane instead of propene.
Decomposition of the EP aliphatic ester is also very efficient via acid-catalysed pathway without
metal assistance to yield ethene and propanoic acid. Addition of Pt to CsPW under H2 causes
hydrogenation of ethene to ethane but does not affect the rate of EP decomposition. Nevertheless,
the Pt-CsPW bifunctional catalyst under H2 shows much better performance stability in EP
iv
decomposition in comparison to the CsPW acid catalyst. This can be attributed to reduction of
catalyst coking in the presence of Pt and H2.
Kinetics of the acid-catalysed decomposition of DPE and EP was studied with a wide range of
tungsten HPA catalysts. Good linear relationships between the logarithm of turnover reaction
rate (TOF) and the HPA catalyst acid strength represented by ammonia adsorption enthalpies
were obtained, which can be used to predict the activity of other Brønsted acid catalysts in these
reactions.
The main results obtained in this thesis are disseminated in the following publications and
conference presentations:
Published papers:
1. K. Alharbi, E. F. Kozhevnikova, I. V. Kozhevnikov, Appl. Catal. A 504 (2015) 457.
2. O. Poole, K. Alharbi, D. Belic, E. F. Kozhevnikova, I. V. Kozhevnikov, Appl. Catal. B 202
(2017) 446.
3. K. Alharbi, W. Alharbi, E. F. Kozhevnikova, I. V. Kozhevnikov, ACS Catal. 6 (2016) 2067.
Poster presentations:
1. K. Alharbi, E. F. Kozhevnikova, I. V. Kozhevnikov, Hydrodeoxygenation of methyl
isobutyl ketone (MIBK) over bifunctional metal-acid catalyst in the gas phase, Poster Day,
University of Liverpool, Liverpool, UK, 10th April, 2014.
2. K. Alharbi, E. Kozhevnikova, I. V. Kozhevnikov, Hydrodeoxygenation of methyl isobutyl
ketone (MIBK) over bifunctional metal-acid catalyst in the gas phase, 4th Northern
Sustainable Chemistry (4th NORSC), Huddersfield University, Huddersfield, UK, 23rd
October, 2014.
3. K. Alharbi, E. F. Kozhevnikova, I. V. Kozhevnikov, Hydrogenation of ketones over
bifunctional Pt-heteropoly acid catalyst in the gas phase, 8th Saudi Students Conference
(SSC), Queen Elizabeth II Centre, London, UK, 31st January - 1st February, 2015.
4. K. Alharbi, E. F. Kozhevnikova, I. V. Kozhevnikov, Deoxygenation of ethers and esters
over bifunctional Pt−heteropoly acid catalyst in the gas phase, Catalysis fundamentals and
practice summer school, University of Liverpool, Liverpool, UK, 20th -24th July, 2015.
v
5. K. Alharbi, E. F. Kozhevnikova, I. V. Kozhevnikov, Hydrogenation of ester over
bifunctional Pt-polyoxometalate and acid catalysts in the gas phase, 9th Saudi Students
Conference (SSC), The ICC Birmingham Broad Street, Birmingham, UK, 13th-14th
February, 2016.
6. K. Alharbi, M. A. Alotaibi, E. F. Kozhevnikova, I. V. Kozhevnikov, Hydrodeoxygenation
of biomass-derived ketones over bifunctional metal-acid catalysts in the gas phase,
Designing New Heterogeneous Catalysts: Faraday Discussion, Burlington House, London,
UK, 4th-6th April, 2016.
vi
Acknowledgements
The most important acknowledgment of gratitude I wish to express is to my supervisor, Professor
Ivan Kozhevnikov, for his excellent guidance, support and encouragement during my study. It
was a pleasure working with him.
I would like to express my deepest sense of gratitude for Dr Elena Kozhevnikova for kind
assistance and resolution of all technical issues through my laboratory work. Without her
assistance, this research could not be performed so smoothly and effectively.
I would like also to thank all member at the University of Liverpool, especially the technical
support team in Chemistry Department.
It is a pleasure to record my thanks to all members in my group for their cooperation and sharing
knowledge.
I extend my heartfelt gratitude to my parents for their big support and faith in me through my
study. Thanks for their love.
I would like to express my deepest appreciate to my mother-in-law, my brother Hani and my best
freind Walaa for being with me through my study abroad giving the happiness and motivation to
me.
My special thanks to my husband Wael and my kids. Without their constant support and love
none of this would have been possible.
Finally, I would like to acknowledge the financial support of the King Abdulaziz University. I
also appreciate the financial management of the Saudi Arabia Cultural Bureau in the UK.
vii
Abbreviations
HDO Hydrodeoxygenation
HPA Heteropoly acid
HPW Tungstophosphoric acid (H3PW12O40)
HSiW Tungstosilicic acid (H4SiW12O40)
CSPW Caesium salt (Cs2.5) of tungstophosphoric acid (H3PW12O40)
POM Polyoxometalate
BET Brunauer-Emmett-Teller method
TGA Thermogravimetric analysis
ICP Inductively coupled plasma
XRD X-ray diffraction
TCD Thermal conductivity detector
STEM Scanning transmission electron microscopy
EDX Energy dispersive X-ray emission
FTIR Fourier transform infrared spectroscopy
GC Gas chromatography
FID Flame ionisation detector
MIBK Methyl isobutyl ketone
MP Methyl pentane
MP-ol Methyl pentanol
DIBK Diisobutyl ketone
EP Ethyl propanoate
DPE Diisopropyl ether
TOF Turnover frequency
viii
Contents
Abstract .................................................................................................................................................. i
Acknowledgements ......................................................................................................................... vi
Abbreviations ................................................................................................................................... vii
Contents............................................................................................................................................. viii
1. Introduction ................................................................................................................................... 1
1.1 Heterogeneous catalysis ..................................................................................................................... 1
1.1.1 Definition and background of catalysis ..................................................................................... 1
1.1.2 Classification of catalytic systems .............................................................................................. 2
1.1.3 Key steps in a heterogeneously catalysed reaction ................................................................... 3
1.1.4 What makes a good catalyst? ...................................................................................................... 4
1.1.5 Catalysis by metals ...................................................................................................................... 5
1.2 Multifunctional catalysis .................................................................................................................... 7
1.3 Heteropoly acids ................................................................................................................................. 9
1.3.1 Definition and structure of HPAs ............................................................................................... 9
1.3.2 Properties of heteropoly acids ................................................................................................. 13
1.3.2.1 Thermal stability of HPAs ................................................................................................. 13
1.3.2.2 Acidic properties of HPAs .................................................................................................. 15
1.3.3 Supported HPAs ........................................................................................................................ 17
1.3.4 Salts of HPAs .............................................................................................................................. 18
1.3.5 HPAs in heterogeneous catalysis .............................................................................................. 20
1.3.6 Metal-HPA multifunctional catalysis ....................................................................................... 22
1.4 Synthesis of biofuels from biomass ................................................................................................. 23
1.5 Deoxygenation of biomass-derived molecules ................................................................................ 26
1.5.1 Introduction ............................................................................................................................... 26
1.5.2 Hydrodeoxygenation of biomass-derived ketones .................................................................. 27
1.5.3 Hydrodeoxygenation of ethers .................................................................................................. 31
1.5.4 Decomposition of esters ............................................................................................................. 34
1.6 Objectives and thesis outline ........................................................................................................... 37
1.7 References ......................................................................................................................................... 39
2. Experimental ............................................................................................................................... 46
ix
2.1 Introduction ...................................................................................................................................... 46
2.2 Materials ............................................................................................................................................ 46
2.3 Catalyst preparation ........................................................................................................................ 47
2.3.1 Preparation of CsnH3-nPW12O40 ................................................................................................ 47
2.3.2 Preparation of Pt, Ru, Cu, Ni and Au modified CsPW .......................................................... 47
2.3.2.1 Preparation of Pt/CsPW ..................................................................................................... 47
2.3.2.2 Preparation of Ru/CsPW ................................................................................................... 48
2.3.2.3 Preparation of Cu/CsPW ................................................................................................... 48
2.3.2.4 Preparation of Ni/CsPW .................................................................................................... 49
2.3.2.5 Preparation of Au/CsPW ................................................................................................... 49
2.3.2.6 Preparation of bimetallic Pt/Au/CsPW catalysts ............................................................. 49
2.3.3 Preparation of carbon-supported metal catalysts ................................................................... 50
2.3.4 Preparation of supported hetropoly acid catalysts ................................................................. 50
2.3.5 Preparation of Nb2O5................................................................................................................. 51
2.3.6 Preparation of ZrO2 .................................................................................................................. 51
2.4 Catalyst characterisation techniques .............................................................................................. 52
2.4.1 Surface area and porosity analysis ........................................................................................... 52
2.4.2 Inductively coupled plasma atomic emission spectroscopy (ICP-AEC) ............................... 54
2.4.3 Powder X-ray diffraction (XRD) .............................................................................................. 54
2.4.4 H2 chemisorption ....................................................................................................................... 55
2.4.5 CO chemisorption ...................................................................................................................... 57
2.4.6 Thermogravimetric analysis (TGA) ......................................................................................... 58
2.4.7 Elemental analysis ..................................................................................................................... 60
2.4.8 Microcalorimetry ....................................................................................................................... 60
2.4.9 Scanning transmission electron microscopy (STEM) with energy dispersive X-ray
emission (EDX) microanalysis ........................................................................................................... 61
2.4.9.1 STEM ................................................................................................................................... 61
2.4.9.2 EDX ...................................................................................................................................... 61
2.4.10 Fourier transform infrared spectroscopy (FTIR). ................................................................ 62
2.5 Catalytic reaction studies ................................................................................................................. 63
2.5.1 Hydrodeoxygenation of biomass-derived ketones .................................................................. 63
2.5.2 Deoxygenation of ethers and esters .......................................................................................... 66
2.6 Product analysis ................................................................................................................................ 66
2.6.1 Gas chromatography ................................................................................................................. 66
2.6.2 GC calibration ............................................................................................................................ 68
2.7 References ......................................................................................................................................... 81
x
3. Catalyst characterisation ...................................................................................................... 83
3.1 Introduction ...................................................................................................................................... 83
3.2 Thermogravimetric analysis ............................................................................................................ 83
3.3 Surface area and porosity studies ................................................................................................... 86
3.4 Metal dispersion of bifunctional catalysts ...................................................................................... 97
3.6 X-ray diffraction ............................................................................................................................. 100
3.7 Fourier transform infrared spectroscopy (FTIR) ....................................................................... 102
3.7.1 Keggin structure ...................................................................................................................... 102
3.7.2 Pyridine adsorption ................................................................................................................. 105
3.8 Microcalorimetry of ammonia adsorption ................................................................................... 106
3.9 Conclusion ....................................................................................................................................... 107
3.10 References ..................................................................................................................................... 109
4. Hydrogenation of ketones over bifunctional Pt-heteropoly acid catalyst in
the gas phase ................................................................................................................................... 111
4.1 Introduction .................................................................................................................................... 111
4.2 Hydrogenation of MIBK over CsPW-supported metal catalysts ............................................... 113
4.3 Dehydration of 2-methyl-4-pentanol over CsPW ........................................................................ 118
4.4 Hydrogenation of aliphatic ketones over Pt/CsPW ..................................................................... 121
4.5 Hydrogenation of acetophenone over Pt/CsPW........................................................................... 123
4.6 Conclusions ..................................................................................................................................... 124
4.7 References ....................................................................................................................................... 126
5. Hydrodeoxygenation of 3-pentanone over bifunctional Pt-heteropoly acid
catalyst in the gas phase: enhancing effect of gold ...................................................... 127
5.1 Introduction .................................................................................................................................... 127
5.2 Effect of gold on HDO of 3-pentanone .......................................................................................... 129
5.3 Catalyst characterisation ............................................................................................................... 140
5.3.1 X-ray diffraction ...................................................................................................................... 141
5.3.2 STEM-EDX .............................................................................................................................. 142
5.4 Turnover rates ................................................................................................................................ 147
5.5 Conclusions ..................................................................................................................................... 149
5.6 References ...................................................................................................................................... 150
6. Deoxygenation of ethers and esters over bifunctional Pt-heteropoly acid
catalyst in the gas phase ............................................................................................................ 152
6.1 Introduction .................................................................................................................................... 152
6.2 Hydrogenation of anisole ............................................................................................................... 153
xi
6.2.1 Catalyst performance .............................................................................................................. 153
6.2.2 Effect of catalyst formulation and preparation on the catalyst performance .................... 157
6.2.3 Proposed mechanism of anisole hydrogenation over Pt-CsPW ........................................... 159
6.3 Decomposition of diisopropyl ether .............................................................................................. 160
6.3.1 Reaction mechanism over Pt-CsPW ...................................................................................... 160
6.3.2 Thermodynamics of DPE decomposition [16] ....................................................................... 161
6.3.3 Decomposition of diisopropyl ether over CsPW and Pt/CsPW ........................................... 163
6.3.4 Effect of temperature on DPE decomposition over CsPW .................................................. 165
6.3.5 Catalyst performance stability over CsPW ........................................................................... 166
6.3.6 Kinetic studies .......................................................................................................................... 166
6.4 Decomposition of ethyl propanoate............................................................................................... 169
6.4.1 Mechanism of acid catalysed decomposition of ethyl propanoate ....................................... 169
6.4.2 Decomposition of EP over CsPW and Pt/CsPW ................................................................... 170
6.4.3 Catalyst performance stability ............................................................................................... 172
6.4.4 Kinetic studies .......................................................................................................................... 175
6.5 Conclusion ....................................................................................................................................... 178
6.6 References ....................................................................................................................................... 180
7. Conclusion ................................................................................................................................... 182
7.1 References ....................................................................................................................................... 187
1
1. Introduction
1.1 Heterogeneous catalysis
1.1.1 Definition and background of catalysis
The word catalysis comes from the Greek prefix of kata-, which means down, and the verb lysein,
which means to break. In 1836, Berzelius introduced the word catalysis in his attempt to describe
the unusual findings discovered by earlier scientists [1]. By definition, a catalyst is a material
that can accelerate the rate of chemical reaction without being substantially consumed in the
reaction [2, 3]. A catalyst acts by reducing the activation energy of the rate limiting step providing
an alternative pathway to avoid the slowest step in the uncatalysed reaction [4].
Various examples of catalysis have been known since ancient times. Perhaps the earliest example
of a catalyst was the use of natural yeasts for the fermentation of the sugar contained in biological
material such as grapes to produce wine and beer [5]. Many applications of catalysis were
developed during the 19th Century. Table 1.1 exhibits some examples of early large scale catalytic
processes with their notable dates [2]. Nowadays, it is estimated that 90% of industrial chemical
processes use catalysts at least at one stage; therefore, catalysis is extremely important
economically [2, 6].
2
Table 1.1 Some large scale catalytic processes [2].
1.1.2 Classification of catalytic systems
In fact, catalytic systems can be divided into two main categories: homogeneous and
heterogeneous catalysis [1, 7, 8]. Homogeneous catalysis occurs when the catalyst and reactant
are in the same phase and thus no phase boundary exists. This can occur either in the gas phase,
for example when using a nitrogen oxide catalyst in the oxidation of sulfur oxide; or in the liquid
phase, such as using acid and base catalysts in the mutarotation of glucose. Another type of
catalytic system is heterogeneous catalysis, which occurs when the catalyst and reactant are in
different phases (gas-solid, liquid-solid or biphasic liquid-liquid).
Most of the large scale industrial catalysis processes are heterogeneous owing to the advantages
of easy catalyst regeneration after reaction, less corrosion and easy catalyst separation form the
reaction mixture [9-11]. Historically, the earliest research on heterogeneous catalysis can be
Reaction
(discoverer)
Catalyst
(date)
2HCl + ½ O2 H
2O + Cl
2
(Deacon)
CuCl2
(1860)
SO2 + ½ O
2 SO
3
(Phillips)
Pt
(1875)
CH4 + H
2O CO + 3H
2
(Mond)
Ni
(1888)
2NH3 + 5/2O
2 2NO + 3H
2O
(Ostwald)
Pt foil
(1901)
C2H
4 + H
2 C
2H
6
(Sabatier)
Pt
(1902)
N2 + 3H
2 2NH
3
(Haber)
Fe
(1914)
3
traced back to the early 19th Century, when Faraday discovered the first heterogeneously
catalysed reaction using platinum for an oxidation reaction [1, 12].
Heterogeneous catalysis in gas-solid and liquid-solid systems is interesting since it presents the
opportunity to deposit and immobilize the active substance on the solid surface. More important,
however, is the difference between the surface and bulk properties. The surface of a solid material
is an abrupt termination of its bulk structure that serves to expose all the surface atoms in an
asymmetric environment. These atoms on the surface have lower coordination than the atoms in
the bulk, hence the surface atoms are ready for interaction with incoming reactant molecules in
order to satisfy their bonding requirements [13]. Since the reaction takes place on the solid
surface of the catalyst, the reactivity of the surface atoms is vital in determining the effectiveness
of the catalyst and the efficiency of a catalytic process.
1.1.3 Key steps in a heterogeneously catalysed reaction
A gas-phase chemical process occurring over a heterogeneous metal catalyst is illustrated
in Figure 1.1. It consists of the following steps [2]:
1) Gas phase diffusion of reactant molecules to the surface of the metal.
2) Adsorption of the molecules to the metal surface.
3) Dissociation of the molecules into atoms may occur on the metal surface
(depending on their internal bond strength).
4) Reaction between dissociated molecules at the surface to form a product; this may
often be the rate-limiting step.
5) Desorption of the product to the gas phase, where the bond between the product
and the surface is broken.
4
Figure 1.1 Molecular and atomic events occurring in a heterogeneous catalytic reaction on a
supported metal catalyst [2].
1.1.4 What makes a good catalyst?
There is a wide range of factors to be taken into account when designing and developing an
appropriate catalyst for a particular reaction process [2, 14]. The first property to be considered
is the active phase of the catalyst, which is vital for kinetic reaction parameters (represented as a
space-time yield). The correct active phase is the fundamental aspect of catalyst preparation.
Secondly, the surface area of the catalyst is very important. A high surface area is generally
required to produce an optimum yield, although, in some cases, a modest surface area is required
to prevent further reactions of the desired products. The longevity and mechanical strength of the
catalyst are also very important parameters in catalyst design. If a catalyst is used commercially,
stability is important for the catalyst to be considered for further development. Catalyst
deactivation can be caused by sintering (reduction of the active phase surface area), or poisoning
(reduction of the active site density) e.g. coke formation on some catalysts through organic
5
reaction. Another factor to be considered in choosing a catalyst is the cost of its production; this
must be low compared to the selling price of the product.
1.1.5 Catalysis by metals
Table 1.2 exhibits various examples where metallic catalysts are used in commercial processes.
The most common metals used in catalytic processes are the transition metals due to their unfilled
d-bands. Almost all metals in the periodic table can be used, however [1, 5].
Table 1.2 Some commercial processes using metal catalysts [5].
Process Catalyst
Ammonia synthesis Promoted iron
Ammonia oxidation Pt/Ru gauze
Fisher-Tropsch synthesis Fe or Co on support
Steam reforming of methane Ni on support (typically Al2O3)
Methanol synthesis Cu/Zn/Al2O3
Methanol oxidation Unsupported silver, BiMo
Reforming of hydrocarbons Pt/Re/support
Automobile exhaust treatment Pt/Pt/CeO2/Al2O3
Selective reduction of NOx in flue-gas V2O5/TiO2/Matrix support
Ethylene oxidation Ag/α-Al2O3
Fat hardening Ni/Al2O3
Although the most common metal catalysts consist of supported metals, there are several
examples of processes that can be carried out over pure metals. The most well-known example
of an unsupported metallic catalyst is the platinum gauze used for ammonia oxidation. Sintering
often occurs in the case of pure metal, however, due to the relatively high temperatures of use,
6
and therefore stabilization of the metal particles is required to achieve the highest possible surface
area. The addition of a promoter can help to achieve this stabilization by anchoring the metal on
the surface and avoiding surface migration. In the case of Pt/Rh gauzes for the oxidation of
ammonia mentioned above, the surface rearrangement is slowed by the addition of Rh on the
surface of Pt. Another example where a promoter is used for metal stabilization is the addition
of alumina in the case of Raney nickel [5].
The surface area of the metallic catalyst is important since the reaction rate is usually determined
by the rate of the surface reaction. It is therefore vital to create the highest possible accessible
metal surface area. For this reason, supported metal catalysts are often used [5].
Preparation methods for supported catalysts have been widely developed to create very small
metal crystallites on high surface area supports. There is a wide range of support materials that
can be used for catalyst preparation [15]. Single oxides such as alumina (Al2O3) or silica (SiO2),
and complex oxides such as silica-alumina or zeolites and active carbons are the most commonly
used supports. In fact, the support itself may not only be an inert material in which the metal is
placed, it can also enhance catalyst performance [5].
Gold has been long considered to be an inactive catalyst; when a suitable preparation method
provides high metal dispersion, however, gold becomes active for various reactions including
CO oxidation. An early application of these catalysts was pioneered by Haruta [16] and
Hutchings [17] who disclosed the peculiarity of the activity of gold in CO oxidation and other
reactions. Since then gold catalysis has become an important topic in the field of catalysis [18].
Recently, several comprehensive reviews and two books have given a good overview of gold
nanoparticles as a catalyst [19-23]. Very good reviews were given by Daniel and Astruc and
Bond, Louis and Thompson, exhaustively covering all aspects related to the preparation and
stabilisation of gold nanoparticles [24].
7
Generally, gold in heterogeneous catalysis is supported on the outer surface of a solid support.
Micro-/meso-porous materials such as silica and metal oxides can be used as hosts in which Au
is combined to increase the total surface area and control the selectivity of the process [24].
1.2 Multifunctional catalysis
Nowadays, much attention is being given to the development of cascade (tandem) processes
using multifunctional catalysts for the production of organic compounds without intermediate
separation. These catalysts contain two or more different active sites working synergistically to
effect several chemical transformations in one pot or on a single catalyst bed [25].
Figure 1.2 shows a step-by-step process in which the starting material A is converted to the final
product D through a three-step reaction which involves the formation of intermediate products B
and C (solid arrows). This process requires the isolation and purification of the intermediates B
and C, making the operation both costly and time consuming. These drawbacks can be overcome,
however, by using a more efficient and environmentally friendly tandem process with a
multifunctional catalyst (broken arrows) [26-28].
A wide range of metal supported bifunctional catalysts are used in heterogeneous catalysis
processes, such as hydrogenation, dehydrogenation and hydroisomerisation reactions [29]. Much
of the research regarding heterogeneous multifunctional catalysis has concentrated on developing
catalysts consisting of transition metals supported on an active support having acidic and/or basic
sites for multistage reactions.
8
Figure 1.2 Multistage organic synthesis where the starting material A is converted to the
desirable product D through B and C intermediates [26]. Broken arrows represent a tandem
process to form the final product without recovery steps after each conversion step.
The group VIII transition metals (e.g. Pt, Pd, Ni, Ru) have been shown to be efficient metal
catalysts in a number of catalytic processes. For example, platinum can be introduced using
different Pt precursors; however, the most commonly used are chloroplatinic acid (H2PtCl6) and
platinum (II) tetramine ion, [Pt(NH3)4]2+ [15].
Different preparation procedures are used to introduce metal precursors onto the support, such
as impregnation, ion-exchange or co-precipitation, followed by drying and calcination followed
by reduction. Calcination has the purpose of decomposing the metal precursor on the support to
achieve the highest metal dispersion [15]. The metal dispersion (metal particle size) can be
affected by the preparation procedures and type of precursor used, and these, therefore, affect the
activity and selectivity of multifunctional metal catalysts.
Conversion steps
Rec
ov
ery s
tep
s
9
Bifunctional metal supported acid/base catalysts have been employed for multistep reactions.
Early work stated that Ni supported on γ-Al2O3 was a good catalyst for the conversion of
propylamine into dipropylamine [30]. Pd/KX has been reported for the one-pot synthesis of 2-
ethylhexanol from n-butanol [31], while Cu supported on Mg(Al)O mixed oxide is used for the
production of isobutanol by coupling MeOH and 2-propanol [32] and a Pd/Mg(Al)O catalyst for
one-pot synthesis of 2-methyl-3-phenyl-propanal from benzaldehyde and propanal [33].
Moreover, many bifunctional catalysts, such as Pd/HZSM-5, have been reported for the one-pot
synthesis of methyl isobutyl ketone (MIBK) from acetone [34-42]. This process consists of three
steps occurring on a single bed containing a bifunctional metal-acid or metal-base catalyst [43].
The first step is the formation of diacetone alcohol (DA) via acid or base catalysed aldol
condensation of acetone. In the second step, DA is converted over an acid catalyst to form mesityl
oxide (MO) and water. Finally, MO is hydrogenated to produce MIBK over a noble metal
catalyst.
Scheme 1.1 Three-step MIBK synthesis from acetone [42].
1.3 Heteropoly acids
1.3.1 Definition and structure of HPAs
Heteropoly acids (HPAs) contain metal-oxygen cluster polyoxometalate anions and protons as
counter cations [44]. The general formula of heteropolyanions is [XxMmOy]q-, where x < m, X is
the heteroatom or central atom, such as P5+, As5+, Si4+, Ge4+ and B3+, and M is a metal ion such
10
as molybdenum(VI), tungsten(VI), vanadium(V), niobium(V) and, less frequently, tantalum(V)
[44].
Heteropolyanions are formed by a self-assembly process in acidified aqueous solution, as shown
below (Equation 1.1) [45]:
23H+ + HPO42- + 12WO4
2- [PW12O40]3- + 12 H2O (1.1)
The first heteropoly compound was discovered by Berzelius in 1826. Since then, a great number
of heteropoly compounds have been synthesised and several assumptions have been made to
clarify their structure [46]. The first X-ray crystal structure of HPA (tungstophosphoric acid,
H3PW12O40) was reported by Keggin in 1933 [47].
HPAs have been found to possess purely Brønsted acidity; and their acidity is stronger than that
of conventional solid acids such as acidic oxides and zeolites. Due to their chemical and physical
properties, HPAs have found various applications, primarily in catalysis as well as in other fields
[44, 48].
HPAs commonly exist as ionic crystals in the solid state (sometimes amorphous), and are
composed of large heteropolyanions (referred to as the primary structure), counter cations, water
of crystallisation and other molecules contained within a three-dimensional arrangement. The
whole arrangement is referred to as the HPA secondary structure. On top of that, the tertiary
structure is a more complex HPA aggregate; this includes the particle size, pore structure and the
proton distribution within the particles [49]. This structure hierarchy is illustrated schematically
in Figure 1.3.
11
Figure 1.3 Primary, secondary and tertiary structures representing the hierarchical
structure of heteropoly acids in the solid state [50].
Structurally, heteropoly acids can be classified into different groups according to the atomic ratio
between the metal atom and heteroatom present. The most common classes are represented in
Figure 1.4 [51, 52].
12
Figure 1.4 Different structural types of HPAs: (a) Keggin, (b) Wells-Dawson and (c) Anderson
structures [52].
Table 1.3 Different structures of HPAs.
Heteropolyanions with the Keggin structure are represented by the formula [XM12O40]x-8. The
Keggin heteropoly compounds (heteropoly acids and heteropoly salts) are more stable and
relatively easily available [45, 46]. The Keggin anion is made up of a central tetrahedron (XO4)
surrounded by twelve edge- and corner-sharing metal-oxygen octahedra (MO6). These octahedra
are arranged in four M3O13 groups, with each group being formed by three octahedra sharing
X/M ratio Chemical formula
(M=Mo or W)
X Structure name
1:12 [Xn+M12O40](8-n)- P5+, As5+, Si4+, Ge4+ Keggin
1:11 [Xn+M11O39](12-n)- P5+, As5+, Si4+, Ge4+ lacunary Keggin
2:18 [X25+M18O62]
6- P5+, As5+, Dawson
1:6 [Xn+M6O24]n- Te6+, I7+ Anderson
13
edges with a common oxygen atom which is also shared with the central tetrahedron XO4, as
shown in Figure 1.4 (a) and Figure 1.5 [46]. The structure contains four different types of oxygen
atoms (Figure 1.5): twelve terminal M=Ot, twelve edge-sharing angular M-Ob1-M shared by the
octahedra within a M3O13 group, twelve corner-bridging quasi-linear M-Ob2-M connecting two
M3O13 groups, and four internal X-Oc-M. It is possible to distinguish between these oxygen atoms
by 17O NMR and fingerprint infrared spectra in the range of 600-1100 cm-1 [45, 46, 53].
Figure 1.5 Localisation of oxygen atoms in the Keggin structure of PW12O403- [54].
The most common examples of the Keggin type heteropoly acids are: 12-phosphotungstic acid
(H3PW12O40), 12-phosphomolybdic acid (H3PMo12O40), 12-silicotungstic acid (H4SiW12O40) and
12-silicomolybdic acid (H4SiMo12O40). These HPAs are commercially available as crystalline
hydrates [45, 46].
1.3.2 Properties of heteropoly acids
1.3.2.1 Thermal stability of HPAs
The thermal stability of heteropoly compounds is a very vital feature for their use in
heterogeneous catalysis. Some HPAs have a fairly high thermal stability in the solid state, up to
14
350 °C, which allows their use as catalysts at moderately high temperatures. Nevertheless, there
is a critical issue about their thermal stability regarding catalyst regeneration. For example,
burning coke that may form on the catalyst surface may require catalyst thermal stability at least
up to 500 oC [46].
The thermal stability is usually determined by thermogravimetric analysis (TGA), differential
thermal analysis (DTA), differential scanning calorimetry (DSC), X-ray diffraction (XRD),
infrared spectroscopy (IR) and solid state NMR. HPAs of the Keggin structure are the most stable
type of HPA. Their stability can be determined in terms of the decomposition temperature at
which all acidic protons are lost (Scheme 1.2) [44, 46, 55].
Scheme 1.2 Thermal decomposition of H3PW12O40 hydrate [55].
As estimated by TGA, the decomposition temperature of the Keggin-type PW, SiW, PMo, and
SiMo decreases in the following order [46, 55]:
H3PW12O40 > H4SiW12O40 > H3PMo12O40 > H4SiMo12O40
465 oC 445 oC 375 oC 350 oC
From TGA under transient conditions, H3PW12O40 showed the highest thermal stability of 465
oC, but under reaction conditions the solid H3PW12O40 catalyst may start to decompose at lower
temperatures than those determined by TGA. [46].
15
1.3.2.2 Acidic properties of HPAs
Reactions catalysed by heteropoly acids take place through the same mechanisms as those
catalysed with conventional Brønsted acid catalysts (Equation 1.2). The substrate (S) is
protonated by proton transfer from the catalyst, and the ionic intermediate is then converted to
yield a reaction product (P) [46]:
S + H+ ⇌ SH+ → P + H+ (1.2)
There are two kinds of proton within crystalline heteropoly acids: (i) hydrated protons
[H(H2O)n]+ and (ii) non-hydrated protons, as represented in Figure 1.6 [46]. The location of
protons in HPAs has been the subject of some discussion [56]. The hydrated protons have a high
mobility, which causes the extremely high proton conductivity of crystalline heteropoly acid
hydrates. The non-hydrated protons, on the other hand, possess considerably less mobility, and
Kozhevnikov has suggested that they are in fact localised on the peripheral oxygen atoms in the
polyanion [46]. In solid HPAs, both hydrated and non-hydrated protons have a role to play in the
formation of the crystal structure, joining the neighbouring heteropoly anions. In crystalline HPA
hydrates, bulk protons exist as plane diaquahydrogen ions. These are quasi-symmetrical
hydrogen bonded species that serve to link the neighbouring heteropolyanions by forming
hydrogen bonds with the terminal W=O oxygens (Figure 1.6 (a) and Figure 1.7) [46].
a
W = O
O = WW = O
O = W
H+
b
W = O
O = WW = O
O = W
H
OH
H+ O
H
H
Figure 1.6 Proposed proton sites in (a) H3PW12O40∙6 H2O and (b) dehydrated H3PW12O40 [46].
16
Figure 1.7 H3PW14O40.6H2O structure represented as two interpenetrating cubic structures [53].
The surface proton sites in solid HPAs are stronger than the bulk proton sites and thus are vital
for heterogeneous acid catalysis. In general, the surface area of crystalline heteropoly acids is
very low (< 10 m2/g) [44-46, 49, 57]. It is suggested that proton sites are localised at the bridging
oxygen atoms in the Keggin unit when the HPA is dispersed on the support so as to enhance the
HPA’s exposed surface area [44].
Heteropoly acids in the solid state possess purely Brønsted acids, and have a stronger acidity,
and therefore higher activity, than conventional acids like SiO2–Al2O3, H3PO4/SiO2, and HX and
HY zeolites [45, 58, 59].
Thermal desorption of basic molecules reveals the acid properties of solid acids. Okuhara et al.
[45] used the thermal desorption of pyridine to compare the acid strength of heteropoly acids and
SiO2-Al2O3. At 300 oC the pyridine molecules adsorbed on SiO2-Al2O3 are completely desorbed,
whereas they remain mostly adsorbed as pyridinium ions on the surface of H3PW12O40 at much
higher temperatures. This indicates that the acidity of H3PW12O40 is much stronger than that of
SiO2-Al2O3.
17
Temperature-programmed desorption (TPD) of ammonia can also be used for acid strength
characterisation [60-61]. Izumi et al. [61] determined the acid strength of HPA in terms of the
temperature of NH3 desorption which decreased in the following order along with decreasing the
acid strength:
H3PW12O40 > H4SiW12O40 > H3PMo12O40 > H4SiMo12O40
592 °C 532 °C 463 °C 423 °C
The acid strength of HPAs can be determined more accurately by the calorimetry of NH3
absorption [63-65]. The order of acid strength thus obtained was the same as that obtained by
ammonia TPD [44, 60, 62]. Usually, the activity of heteropoly acid catalysts is consistent with
this order both in homogeneous and heterogeneous systems [45, 66].
1.3.3 Supported HPAs
Bulk Keggin HPAs have a very low surface area (< 10 m2/g) and are highly soluble in polar
solvents such as water, alcohols and ethers, which limit their activity as heterogeneous catalysts.
These drawbacks can be overcome, however, by supporting HPAs onto high surface area
supports, for example silica. The advantage of dispersing HPAs on high surface area supports is
that the number of active sites on the surface of HPAs may also increase along with thermal
stability, which will enhance their catalytic activity.
The acidity and catalytic activity of supported HPAs greatly depend on the type of support, the
HPA loading and the pre-treatment conditions [44, 46]. Generally, acidic and/or neutral supports,
such as silica [62], active carbon [67, 68], ion-exchange resin [69], etc., are preferred. In contrast,
basic solid supports such as MgO and Al2O3 are not suitable for use as supports, because they
tend to decompose HPAs due to the instability of HPAs in basic aqueous solutions [51, 66, 70].
18
Usually, strong interaction is observed between heteropoly acids and their support at low HPA
loadings, which decreases the acid strength of the HPA [45]. Frequently, silica is used as a
support material for HPAs, which is relatively inert towards HPA. Nevertheless,
microcalorimetry of NH3 adsorption showed that the acid strength of H3PW12O40 decreases when
it is loaded on SiO2 due to the interaction between HPAs and the surface silanol groups, as shown
in Figure 1.8 [71-73].
Figure 1.8 Differential heats of NH3 adsorption onto H3PW12O40 and 20 wt% H3PW12O40/SiO2
determined at 150 °C after catalyst pre-treatment at 300 oC /10-3 mmHg [71].
1.3.4 Salts of HPAs
The chemical formula of Keggin type HPA salts is M1xHy-xM
2M312O40, where M1 is K+, Cs+, Rb+;
M2 is P or Si; M3 is W or Mo; x is commonly 2.5 and y is 3 or 4 if M2 is P or Si, respectively.
The heteropoly salts are prepared by replacing protons in their parent acids with different metal
ions. The nature of the counter cations in HPA salts is very important for their acidity, porosity,
19
solubility and thermal stability [44, 49, 74]. The HPA salts can be classified according to the size
of their counter cations into two groups [75].
Group I: those with small counter cations such as Li+, Na+:
• low surface area (under 10 m2 g-1),
• high solubility in water,
• absorption capability of polar or basic molecules in the solid bulk.
Group II: Large monovalent cations, such as NH4+, K+, Cs+:
• high surface area (over 100 m2 g-1),
• water-insoluble,
• unable to absorb polar molecules in the bulk.
The HPA salts with large cations like Cs+ are water-insoluble and have a surface area exceeding
100 m2g-1 [76, 77]. In contrast to alkali-exchange zeolites, the partially substituted Cs salts of
HPA have strong surface acidity [49, 78].
Okuhara et al. have reported the effect of Cs substitution on the surface area of H3PW12O40
(Figure 1.9) [45]. The pore size of CsxH3-xPW12O40 can be precisely controlled by its Cs content.
The number of surface protons decreases when the Cs content, x, in CsxH3-xPW12O40 increases
from 0 to 2 due to the reduction in the surface area, but sharply increases when x further increases
from 2 to 3. The surface area and surface acidity reach a maximum at x = 2.5. When x increases
above 2.5, the surface acidity dramatically reduces since the formal amount of protons becomes
very low [45].
HPA salts are normally more stable than their parent acids. For instance, Cs2.5H0.5PW12O40 starts
to decompose at 500 °C, whereas the parent acid H3PW12O40 decomposes at the relatively lower
temperature of 300 °C. The relative stability of HPA salts, meanwhile, depends on the type of
20
counter cation and increases in the following order: Ba2+, Co2+ < Cu2+, Ni2+ < H+, Cd2+ < Ca2+,
Mn2+ < Mg2+ < La3+, Ce3+ < NH4+ < K+, Tl+, Cs+ [49].
The Keggin heteropoly salts are meso-microporous materials which allow other chemical
functions, such as metal functionality, to be introduced. The metal particle size and dispersion
can be controlled by the metal loading in HPA salts. In this respect, it has been found that
introducing 0.5 wt% Pt in Cs2.5H0.5PW12O40 did not affect the pore size of this salt, and the
platinum particle size was less than 10 Å. The Keggin HPA salt CsxH3-xPW12O40 (especially
when x is equal to 2.5) modified with Pt is a promising metal-acid bifunctional catalyst for the
conversion of renewable resources to chemicals and fuels [74, 79].
Figure 1.9 Surface area and surface proton density of CsxH3-xPW12O40 as a function of Cs
content [45].
1.3.5 HPAs in heterogeneous catalysis
Heteropoly acids have found numerous applications as catalysts in heterogeneous gas-solid and
liquid-solid systems [46, 49, 50, 57, 80]. The most important advantage of heterogeneous systems
is that the catalyst can be easily separated from the reaction mixture and reused. There is a critical
issue, however, in the form of HPA’s relatively low thermal stability regarding catalyst
21
regeneration; for example, for burning coke that may form on the catalyst surface thus reducing
catalyst life [44, 46, 55].
Misono et al. [45, 49, 50] have classified heterogeneous HPA catalysis into three types: surface-
type, bulk type I (pseudo-liquid) and bulk type II, as illustrated in Figure 1.10.
Figure 1.10 Three types of catalysis by solid heteropoly compounds [50].
The surface-type catalysis is a conventional acid or oxidation heterogeneous catalysis which
takes place on the surface of a solid catalyst, i.e. on the outer surface and pore walls. The reaction
rate in this case is proportional to the catalyst surface area. An example of this type is the
oxidation of aldehydes and CO.
The bulk type I occurs in the conversion of a polar substrate (e.g. alcohol) with a bulk solid
heteropoly acid or a soluble heteropoly salt (i.e., salts with small metal cations such as Li+, Na+,
etc.) at low temperature. In this case, the substrate is absorbed into the catalyst bulk, penetrating
in between the polyanions and reacting there, so the catalyst performs like a concentrated solution
22
(pseudoliquid phase). In this type of heterogeneous catalysis, the surface and bulk acid site
contribute in the reaction. The reaction rate is proportional to the catalyst volume (catalyst
weight). Dehydration of lower alcohols at low temperature is suggested to occur by this
mechanism.
The bulk type II catalysis, meanwhile, occurs in oxidation catalysis at high temperatures,
accompanied by the migration of redox carriers (protons and electrons) in the solid bulk, and
with the whole of that solid bulk taking part in the redox cycle. In this type of catalysis the
reaction rate is expected to be proportional to the catalyst weight.
1.3.6 Metal-HPA multifunctional catalysis
The diverse physicochemical properties of HPAs allow for their use as multifunctional catalysts.
In addition to the acid and redox properties of HPA, it is possible to introduce other chemical
functions such as metal functionality [44-46]. Nevertheless, only a few studies have used HPAs
as multifunctional catalysts in multistep reactions [41].
As mention above, some acidic heteropoly salts, e.g. CsxH3-xPW12O40, possess strong proton
acidity and a relatively high surface area. These have therefore been used in metal-acid
bifunctional catalysts as an acidic support for some metals [50, 81]. For example, Pd-modified
Cs2.5H0.5PW12O40 (CsPW) was used for one-pot synthesis of methyl isobutyl ketone (MIBK)
from acetone [41]. This reaction occurs in three consecutive steps, as mentioned in section 1.2.
Moreover, Ru supported on CsPW has been used to form propanediol by glycerol hydrogenolysis
[82]. Alotaibi et al. [83] have studied the deoxygenation of propanoic acid using bifunctional
metal-acid catalysis. They reported that Pd and Pt modified CsPW is an efficient catalyst for
propanoic acid decarbonylation to produce ethane.
The Pd-H3PW12O40/SiO2 catalyst has been employed for one-step synthesis of (-)-menthol from
citronellal [84]. This process comprises two steps, the production of isopulegol from cyclisation
23
of (+)-citronellal occurring over acid sites of the heteropoly acid, followed by isopulegol
hydrogenation over metal sites (Pd) leading to the formation of menthol, as shown in Scheme
1.3. Supporting Pd-H3PW12O40 on silica is important to ensure heterogeneity of the catalyst and
provide a high catalyst surface area.
Scheme 1.3 One-step synthesis of (-)-menthol from citronellal over Pd-H3PW12O40/SiO2 [84].
Kozhevnikov et al. have reported that doping HPAs with platinum group metals enhances
catalyst regeneration by coke burning. In Pt- and Pd-modified HPA catalysts only soft coke was
formed, and the catalyst could be regenerated by burning coke off at 350 °C without destroying
the structure of the HPA [85, 86].
1.4 Synthesis of biofuels from biomass
Before discovering fossil fuels, the world’s energy demands were met by using plant biomass.
Currently, the main source of energy is fossil fuels, namely coal, natural gas and petroleum. This
energy pool was established in the 19th Century and helped to develop the high modern standards
of living [87]. Fossil fuels provide over 80% of the world’s energy consumption [88]. These days
it is important to find alternative sources of energy and raw materials to eliminate the imbalance
in fossil fuel sources. In this respect, attention has been given to plant biomass, which, in contrast
24
to fossil sources, is a renewable energy source. Biofuels produce considerably less greenhouse
gas emissions than do fossil fuels and can even be greenhouse gas neutral if efficient processes
of conversion of biomass to liquid fuel are developed [87, 89, 90].
The total annual biomass resources reach 1.5 trillion tons [91] and increase annually by 100
billion tons owing to photosynthesis [92]. Only a limited amount of plant biomass is used as an
alternative source of energy, however. Nowadays, biomass amounts to about 10-12% of world
energy consumption [88]. Experts have predicted that by 2030, 20% of transportation fuel and
25% of chemicals will be produced from biomass [87].
Currently, the production of liquid biofuels is based on the transformation of agricultural plants
into bioethanol and biodiesel. In Brazil, ethanol production relies on the juice from sugar cane. In
the USA, the main source of ethanol production is starch from corn [93]. These agricultural plants,
however, are required in the food industry, which has limited their application in the production
of liquid biofuel [94[. It is thought that there is not enough agricultural land available in the world
to grow sufficient energy crops to replace conventional fuel with biofuels. Non-edible biomass is
an alternative source of biofuel without competing with the food industry [95]. This can be
obtained from wood and other plant species (such as Bermuda grass and switchgrass), plant
residues (such as cobs, stalks and husks), residues of forest (such as twigs), etc.
The cheapest and most abundant source of biomass is lignocellulose. This is a promising raw
material for the production of transportation biofuel. Bio-oil can be produced by pyrolysis or
liquefaction of lignocellulose. The conversion of this material to liquid transportation fuel
requires the removal of some or all oxygen as CO2 and H2O to form molecules with desirable
properties for combustion [87, 96].
25
Lignocellulose is composed of a carbohydrate polymer, cellulose (a crystalline glucose polymer),
hemicellulose (a complex amorphous polymer), and lignin (a polyaromatic compound) (Figure
1.11) ([87] and references therein).
Figure 1.11. Biomass composition [87].
The chemical formula of cellulose is (C6H10O5)n, consisting of a linear chain of D-glucopyranose
connected via β-1,4-glycosidic linkages, usually in the crystal form with an extended flat 2-fold
helical configuration. Hydroxyl groups of glucose form hydrogen bonds that help to maintain
and strengthen the flat linear configuration of the chain. This interconnected molecular structure
makes cellulose chains completely hydrophobic. A hydrogen bond is formed between the OH
groups on the glucose, with oxygen atoms on the same or on a neighbouring chain clasping the
chains together and making microfibrils with high tensile strength [87].
26
Hemicellulose is a sugar polymer that usually constitutes 20-40 wt % of biomass. In contrast to
cellulose, which is a polymer of only glucose, hemicellulose is a polymer of five different sugars
containing both pentose (usually D-arabinose and D-xylose) and hexose (D-mannose, D-glucose
and D-galactose) units. The degree of polymerization of cellulose is approximately 10000 to
15000 units, whereas the polymerization degree of hemicellulose is only 50 to 200 monomer
units. This means that hemicellulose decomposes more easily than cellulose. Xylan is the most
common type of hemicellulose, and consists of a xylose unit connected via a β-1,4-glycosidic
linkage [87].
Lignin is an amorphous polyaromatic hydrophobic compound, which constitutes 10-25 wt% of
biomass. Lignin is found in the cell walls of certain biomass, particularly woody biomass. It is a
cross-linked three-dimensional polymer formed by a phenylpropane unit. Figure 1.11 shows the
three main monomer units of lignin (coumaryl, coniferyl and sinapyl alcohol). Lignins of
softwoods are mainly formed from coniferyl alcohol, whereas lignins of hardwoods have both
coniferyl and sinapyl alcohol as monomer units. Grass lignin usually consists of all three types
of the phenylpropane units. Lignin’s complex structure makes it hard to decompose using
chemicals and microorganisms. All of these lignocellulose types are found universally in
different kinds of biomass and are the most abundant organic matter on the earth [87].
1.5 Deoxygenation of biomass-derived molecules
1.5.1 Introduction
Oxygen-containing organic compounds such as ketones, carboxylic acids, alcohols, phenols, etc.,
are readily available from natural resources, and are attractive as renewable raw materials for the
production of value-added chemicals and bio-fuels [96, 97]. For fuel applications, they require a
reduction in oxygen content to increase their caloric value. Much current research is therefore
focussed on the deoxygenation of organic oxygenates using heterogeneous catalysis, in particular
27
for the upgrading of biomass-derived oxygenates obtained from fermentation, hydrolysis and fast
pyrolysis of biomass [98-101].
Traditional reduction methods such as Clemmensen and Wolff–Kishner reduction require very
drastic reaction conditions and produce large amounts of by-products [102].
Hydrodeoxygenation (using H2 as the reductant), on the other hand, is considered to be the most
effective method for the deoxygenation of oxygen-containing compounds [83, 103-105].
Catalytic bio-oil hydrodeoxygenation involves treating bio-oil at high temperatures and high
hydrogen pressure in the presence of a catalyst. Furmisky [106] and Bu et al. [107] have reviewed
heterogeneous catalysis for hydrodeoxygenation. Industrially, sulfided CoMo and NiMo
catalysts are commonly used for the removal of oxygen, sulfur and nitrogen from petroleum
fuels. Noble metals such as Pd, Pt and Ru can also be used for hydrodeoxygenation [87, 107,
108].
Despite the fact that several types of catalysts such as CoMo and NiMo have been used
industrially, these catalysts readily undergo deactivation owing to coke deposition. Moreover,
these methods involve very drastic reaction conditions (up to 300 °C) [105].
These drawbacks can be overcome by the one-pot strategy using metal-acid bifunctional
catalysts. This, however, requires more effort to improve such catalysts in respect to the scope
of the substrates, reaction temperature and pressure, as well as the hydrogen consumption [103-
105, 109].
1.5.2 Hydrodeoxygenation of biomass-derived ketones
Biomass-derived ketones can be further upgraded by aldol condensation and hydrogenation to
produce alkanes that fall in the gasoline/diesel range. The hydrogenation of ketones on supported
metal catalysts (e.g. Pt/C and Pd/C) to form alcohols is feasible and well documented [110],
however, further hydrogenation to alkanes is rather difficult to achieve on such catalysts [103,
28
104]. The ketone-to-alkane hydrogenation can be achieved much more easily using bifunctional
metal-acid catalysts ([103-105] and references therein).
Ketones can be obtained from the ketonisation of carboxylic acids. In this process, two
molecules of a carboxylic acid react to produce ketone (Equation 1.3) [111, 112]. This reaction
allows for the partial deoxygenation of carboxylic acids and the further upgrading of their
backbone.
2 RCOOH R2CO + CO2 + H2O (1.3)
A variety of acidic and basic metal oxide and mixed oxide catalysts have been used for
ketonisation [111-115]. Basic sites are more favourable for this type of reaction, however [112].
Industrially, aldol condensation is employed for the transformation of acetone into C6, C9 and
larger organic molecules such as MIBK and diisobutyl ketone (DIBK) (Scheme 1.4), which are
used as solvents in paints, coatings and resins. Many bifunctional catalysts have been reported
for the single stage conversion of acetone to MIBK, as described in section 1.2. Multifunctional
catalytic systems reported in the literature include Pd supported on cation exchange resins and
on zirconium phosphate [116], Pd on zeolites [38], Pd on Cs2.5H0.5PW12O40 [41], Pd on ZnCr
mixed oxide [40], Pd on MgO/SiO2 [117], Pt, Pd, Ni and copper on activated carbon [118, 119],
Pd on ALPO4-11 and SAPO-11 [39].
Scheme 1.4 Formation of MIBK and DIBK from acetone [55].
29
Zaccheria et al. studied the deoxygenation of aromatic ketones into bicyclic ethers in the liquid
phase. They used Cu/SiO2–Al2O3 in the deoxygenation of fluorenone at 90 0C and 1 atm H2. This
catalyst showed very low selectivity towards the corresponding methylene formation (48%) but
Cu/SiO2–ZrO2 was found to be an effective catalyst (with 91% selectivity). They also found that
acidic support was not necessary, and that the Cu/SiO2 catalyst was able to reduce aromatic
ketones with 100% selectivity [120].
Hydrogenation of acetophenone has been studied by Jiang and co-workers using a PtxPdy/ZrO2
catalyst, where x and y correspond to the atomic ratios of Pt and Pd, respectively. Here, the
catalyst was used for the solvent-free hydrogenation of ketones at 140 0C and 60 bar hydrogen
[121].
Bimetallic Pt–Pd/ZrO2 catalysts showed excellent catalytic performance in the total removal of
oxygen from acetophenone to produce ethylbenzene (EB), but Pd/ZrO2 exhibited greater EB
selectivity (64%) than Pt/ZrO2 (14%). For bimetallic catalysts, the selectivity toward EB
increased as the content of Pd increased, and when the Pt content was increased, the selectivity
of Pt–Pd/ZrO2 to phenyl ring hydrogenation products also increased (Scheme 1.5). Overall, it
seems that Pd sites favour hydrogenation of the carbonyl group while Pt sites help the phenyl
group hydrogenation. Jiang et al. also reported that Pt50Pd50/ZrO2 showed the highest activity
(about 85% conversion with 50% EB selectivity) [121].
30
Scheme 1.5 Hydrogenation of acetophenone over bimetallic Pt–Pd/ZrO2 catalysts [121].
Pt–Pd/ZrO2 catalysts have also been used in cyclopentanone hydrogenation. During the
conversion of cyclopentanone, only cyclopentanol was formed, and no cyclopentane was
obtained [121].
Alotaibi et al. [104] investigated a number of metal catalysts (Pd, Pt and Cu) supported on silica
and active carbon for MIBK hydrogenation in the temperature range of 100–400 0C. 10% Pt/C
and 10% Pd/C catalysts showed the highest selectivity to 2-methylpentane (87 and 94%
respectively) at a temperature as high as 300 0C, which indicates that further hydrogenation of 2-
methylpentanol to 2-methylpentane (2MP) is rather difficult to achieve on such catalysts.
This group has also used bifunctional metal-acid catalysts to study the hydrodeoxygenation of
MIBK to produce alkane in one step on a single catalyst bed using platinum metals supported on
zeolites such as H-ZSM-5, H-Beta and H-Y. The Pt/H-ZSM-5 catalyst exhibited the best
performance, giving >99% selectivity to methylpentanes (2MP/3MP = 83:17) at 100% MIBK
conversion at 200 0C [104].
31
The hydrodeoxygenation of MIBK has also been studied using bifunctional metal-
polyoxometalate catalysts comprising Pt, Pd, Ru, or Cu supported on a Keggin heteropoly salt
Cs2.5H0.5PW12O40 (CsPW) [103]. At 100 0C, 0.5%Pt/CsPW catalyst showed very high activity
giving ≥99% MIBK conversion with 100% 2MP selectivity. It has been suggested that MIBK-
to-2MP hydrogenation over Pt/CsPW at 100 0C is limited by the first step, i.e., the hydrogenation
of MIBK to MP-ol (Scheme 1.6). This is mainly based on the fact that the reaction rate scales
with Pt loading, while 2MP selectivity remains constant at ∼100% [103].
Scheme 1.6 MIBK hydrodeoxygenation via bifunctional metal-acid catalysis [103].
Ketones, such as MIBK, acetone, butanone, cyclohexanone, pentanone, etc., can be
hydrogenated to produce alkanes, 2-methylpentane, propane, pentane, etc. C5+ alkanes are in the
gasoline range and could therefore be blended with the straight-run gasoline, and subjected to
standard catalytic reforming [122] to enhance their octane numbers for use through the existing
fuel infrastructure.
1.5.3 Hydrodeoxygenation of ethers
While methanol and ethanol have higher octane numbers and are cheaper than their ethers, they
have the drawback of being water-miscible, with a low Reid vapour pressure. Since the 1990s,
therefore, attention has been drawn to ethers [123].
Presently, all major octane enhancing ethers are obtained from the reaction of the C1 to C3
alcohols with C4 or C5 tertiary olefins (etherification reaction) [123]. Diisopropyl ether (DIPE)
can be easily obtained from the base olefin, propylene and water. Commercially, DIPE is only
32
produced as a by-product of isopropanol manufacture and, to the author’s knowledge, no
industrial scale commercial plants have thus far been commissioned [123]. Many patents,
however, claim to achieve DIPE synthesis using propylene and water as feedstocks in a two-
stage process [124], a one stage process [125], or by reactive distillation [126]. Texaco [127], on
the other hand, proposed a two stage process of DIPE synthesis using acetone and hydrogen as
starting materials. In all these processes, DIPE is eventually formed throw the intermediate
isopropanol. The reactions taking place in the system are shown in Scheme 1.7 [123].
Scheme 1.7 Diisopropyl ether synthesis [123].
Isopropanol is produced as an intermediate either by acetone hydrogenation (step I-B), as in the
case with Texaco [127], or by the conventional method of the hydration of propylene (step I-A).
After that, this intermediate reacts further to produce diisopropyl ether by either etherification
with propylene (step II-A) or by dehydrative etherification (step II-B) [123].
In addition to the more widely studied conversion of cellulose-derived oxygenates, attention has
been given to the production of liquid hydrocarbon fuels from lignin-derived components [106,
107, 128-135]. Lignin is a large polyaromatic compound constituting up to 30% biomass; this
means that phenolic compounds represent a significant fraction of the biomass of pyrolysis bio-
oil. In contrast to the higher oxygen content and shorter carbon chains of the cellulose-derived
oxygenates, phenolics have a carbon chain number already in the gasoline range, but their lower
33
oxygen content means that the deoxygenation process must still preserve the carbon number
within the range of gasoline. Methoxy and hydroxy are the most important functional groups of
phenolics in bio-oil. The effect of the delocalization of the oxygen lone pair orbital onto the π
orbital of the aromatic ring reinforces the bond between C (aromatic) and oxygen of the
phenolics, causing a higher energy barrier than that for the C (aliphatic)-O bond for oxygen
removal [106]. Challenges still remain, therefore, in terms of finding an efficient catalyst for the
deoxygenation of phenolics with maximised carbon retention in liquid fuels [136].
Zhu et al. [130] investigated gas phase transalkylation and hydrodeoxygenation of anisole over
a Pt/H-Beta catalyst at 400 oC and atmospheric pressure. This catalyst showed very high
conversion, but suffered from rapid deactivation. They also compared the product gained on the
bifunctional catalyst with that produced on two monofunctional catalysts (Pt/SiO2 and H-Beta).
This comparison showed that the acid function accelerates the methyl transfer reaction
(transalkylation) from methoxyl to the phenolic ring. The metal function accelerates
demethylation, hydrodeoxygenation and hydrogenation in this order. On a Pt/H-Beta catalyst,
meanwhile, both hydrodeoxygenation and methyl transfer occurred, resulting in toluene, benzene
and xylenes. In comparison with the monofunctional catalysts, bifunctional Pt/H-Beta increases
catalyst stability in respect to deactivation and reduces coke deposition.
Hydrodeoxygenation of an aqueous mixture of bio-derived phenolic monomers to hydrocarbon
and methanol has also been studied using metal catalysts (Pd and Ni) in the presence of acid
(H3PO4 or Nafion/SiO2) at 200 oC. A Raney Ni catalyst with Nafion/SiO2 showed good activity
and selectivity, with nearly 100% yields. Raney Ni acts as the hydrogenation metal catalyst and
Nafion/SiO2 acts as the Brønsted solid acid for hydrolysis and dehydration [131].
Lee et al. [132] tested the hydrodeoxygenation of lignin monomer guaiacol over bifunctional
catalysts comprised of noble metals supported on the acidic matrices, Rh/SiO2-Al2O3 and
34
Rh/ZrO2, in the temperature range of 220-310 oC. They demonstrated that the selectivity to
cyclohexane increases with increasing temperature.
Zhao et al. [133] reported the use of a bifunctional metal-acid catalyst (Pd/C (5 wt%) and H3PO4-
H2O (0.5wt%-80 ml)) in aqueous phase hydrodeoxygenation of anisole at 150 oC and 5 MPa H2.
The main product was cyclohexanone (80% selectivity) produced by hydrogenation of phenol,
which indicates that under the chosen conditions, hydrolysis of anisole to phenol is the
dominating reaction.
More recently, Ni-based catalysts were used to study the effect of the metal-support interaction
on the selective anisole hydrodeoxygenation to aromatics. Ni-containing (20 wt% loading)
catalysts supported on SBA-15, Al-SBA-15, γ-Al2O3, microporous carbon, TiO2 and CeO2 were
tested at 290-310 oC, 3 bar hydrogen pressure and space velocity (20.4 and 81.6 h-1). At 310 oC
and 20.4 h-1 space velocity, the Ni/C catalyst showed the best activity and selectivity toward
benzene (64% yield) owing to the strong acidity and good metal dispersion. The findings suggest
that selecting appropriate catalyst characteristics can promote the selective production of
aromatics from biomass in a bio-refinery scheme [134].
Finally, bifunctional Pt supported on HY zeolite showed high activity and selectivity for the
hydrodeoxygenation of phenol at 250 oC in a fixed-bed reactor and high H2 pressures forming
hydrocarbons, some with enhanced molecular weight [135].
1.5.4 Decomposition of esters
Ethyl propanoate is produced from the esterification of propanoic acid with ethanol.
Hydrodeoxygenation can upgrade this ester to enhance fuel properties and to gain synthetic
biofuels [137].
35
Ethyl propanoate is used as a solvent in perfumery and fragrance formulations. It can also be
used to manufacture various propanoates for use in pharmaceuticals, antifungal agents,
agrochemicals, plasticizers, rubber chemicals, dyes, etc. [137].
Mechanistically, the acid-catalysed decomposition of ethyl propanoate (EP), an aliphatic ester,
involves ester protonation to form an oxonium ion followed by acyl−oxygen or alkyl−oxygen
bond breaking, which can occur through monomolecular (AAC1 or AAL1) or bimolecular (AAC2
or AAL2) pathways (Scheme 1.8). This mechanism is well documented for acid-catalysed
hydrolysis of esters in homogeneous solutions [138]. In the gas phase, due to the lack of solvation
of cationic intermediates (acylium and primary alkylcarbenium ions), the acid-catalysed EP
decomposition yielding an equimolar mixture of propanoic acid and ethene (this will be discussed
in more detail in Chapter 6).
Scheme 1.8 Mechanism of acid ester hydrolysis (when R3 = H). In the case of R3 = alkyl, it occurs
through transesterification for the AAC mechanism and through etherification for the AAL
mechanism [139].
36
A ruthenium–platinum bimetallic catalyst supported on boehmite and γ-Al2O3 was used to study
the hydrogenation of ester to produce alcohol in the liquid phase. The effect of surface hydroxyl
groups on support and solvent was also reported. In an aqueous solution the Ru–Pt/AlOOH
catalyst exhibited better performance than the Ru–Pt/γ-Al2O3 in respect to the hydrogenation of
methyl propanoate. At 180 oC and 5 MPa of H2 pressure, Ru–Pt/AlOOH catalyst showed very
good activity, giving 89 % methyl propanoate conversion with 98% 1-propanol selectivity. The
good catalyst performance is attributed to the cooperation between the hydroxyl groups of
ALOOH surface and water solvent [140].
Senol et al. [141] have studied the hydrodeoxygenation of aliphatic ester methyl heptanoate in
the liquid phase over sulfided NiMo/γ-Al2O3 and CoMo/γ-Al2O3 catalysts at 250 0C and 1.5 MPa
of H2. They also studied the effect of water on the activity of these catalysts. Moreover, they
examined the addition of H2S to the feed, alone and simultaneously with water. They found that
under the same conditions the NiMo catalyst showed much higher activity than the CoMo
catalyst. They also reported that the addition of water decreased the activity of the catalyst,
however, the conversion increased with the addition of H2S to the same level as that without
water addition. The highest ester conversion was obtained when only H2S was added. In addition,
the hydrocarbon yield decreased with an increase in the amount of water, while the concentration
of oxygen-containing intermediates increased. The addition of H2S enhanced the selectivity
toward C6 hydrocarbons, but the catalysts suffered from deactivation.
Zhang et al. [142] investigated the hydrogenation of ethyl acetate to produce ethanol over Ni-
based catalysts prepared from Ni/Al hydrotalcites. The highest selectivity (68%) and yield (62%)
of ethanol was obtained using a RE1NASH-110-3 catalyst at 250 °C and 6 MPa of hydrogen
pressure.
37
The present work demonstrates that Pt/Cs2.5H0.5PW12O40 is an efficient catalyst for gas-phase
hydrodeoxygenation of a variety of oxygen-containing compounds such as ketones, ethers, and
esters in a fixed-bed reactor at temperatures below 100 oC and ambient H2 pressure (Chapters 4-
6). More recently, Mizuno et al. [105] have applied this catalyst for hydrogenation of ketones,
phenols, and ethers in the liquid phase in a batch reactor at 120 oC and 5 bar H2 pressure. This
catalyst also showed good activity and selectivity under such conditions.
1.6 Objectives and thesis outline
The conversion of biomass-derived oxygenated molecules, such as ketones, ethers, esters, etc.,
into value-added chemicals and bio-fuels has been attracting increasing attention, as a result of
the decline in oil resources and global warming. For fuel applications, these oxygenates require
their oxygen content to be reduced so as to increase their caloric value. The aim of this study is
to examine the gas phase hydrodeoxygenation of a wide range of oxygenated compounds,
ketones, ethers and esters, over bifunctional metal acid catalysis under mild conditions. The
metals used include Pt, Ru, Ni and Cu supported on Cs2.5H0.5PW12O40 (CsPW), an acidic Cs salt
of a Keggin-type heteropoly acid H3PW12O40. Details about the reaction mechanisms are studied,
as well as the effect of various catalyst preparation methods.
Another target of this study is to investigate the effect of a gold additive on the activity and
performance stability of physically mixed and supported bifunctional catalysts comprising Pt and
CsPW in HDO of 3-pentanone in the gas phase.
The catalysts are characterised using various techniques to compare properties that are vital to
their use in the reaction. The metals supported on the surface of Cs2.5PW and active carbon are
probed using scanning transmission electron microscopy (STEM), X-ray diffraction (XRD) and
gas chemisorption for the purpose of determining their dispersion and average particle size. Other
38
techniques utilised are inductivity coupled plasma (ICP), Fourier transform infrared spectroscopy
(FTIR), ammonia adsorption microcalorimetry and element analysis (C, H analysis).
Chapter 1 provides a general introduction to heterogeneous and multifunctional catalysis, along
with a brief discussion of the structure and properties of heteropoly acids. Recent literature on
the deoxygenation of oxygen-containing organic compounds to form value-added chemicals and
biofuels is also covered.
Chapter 2 provides a description of the methods for preparing the bifunctional, acid and
bimetallic catalysts, along with the techniques that are used for the characterisation of catalysts
and gas phase catalyst reaction testing.
Chapter 3 details the results of the catalyst characterisation techniques, focussing especially on
the properties that will have a bearing on the catalytic performance during the
hydrodeoxygenation (HDO) of oxygenated compounds.
Chapter 4 investigates the catalytic performance of metal/CsPW catalysts in the HDO of a
variety of ketones, including aliphatic ketones and acetophenone in a gas phase reaction operated
under mild conditions. This provides insights into the reaction mechanism.
Chapter 5 reports the enhancing effect of gold in the HDO of ketone, 3-pentanone, over a
bifunctional Pt/CsPW catalyst in the gas phase.
Chapter 6 explores the deoxygenation and decomposition of a series of ethers and esters,
including the aromatic ether anisole, the aliphatic diisopropyl ether (DPE) and the aliphatic ester
ethyl propanoate (EP) in the gas phase using bifunctional metal-acid catalysis with the main focus
on the Pt−CsPW catalyst. Moreover, the relationship between the turnover reaction rate (turnover
frequency) and the HPA acid strength is discussed.
Chapter 7 draws conclusions from the key findings from the previous chapters.
39
1.7 References
1. G. C. Bond, Heterogeneous Catalysis: Principles and Applications, Clarendon Press,
Oxford, 1974.
2. M. Bowker, The Basis and Applications of Heterogeneous Catalysis, Oxford
University Press Inc., New York, 1998.
3. B. C. Gates, Catalytic Chemistry, Wiley, Chichester, 1992.
4. M. Michael, Principles of Chemistry, W.W. Norton & Company, 2000.
5. J. R. H. Ross, Heterogeneous Catalysis: Fundamentals and Applications, Elsevier:
Amsterdam, 2012.
6. J. M. Thomas, W. J. Thomas, Principles and Practice of Heterogeneous Catalysis,
VCH, Weinheim, 1997.
7. O. Deutschmann, H. Knözinger, K. Kochloefl, T. Turek, Heterogeneous Catalysis
and Solid Catalysts, Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH
Verlag GmbH & Co. KGaA, New York, 2000.
8. G. Ertl, H. Knözinger, J. Weitkamp, Handbook of heterogeneous catalysis, Wiley-
VCH, Weinheim, 1997.
9. H. Hattori, Mat. Chem. Phys. 18 (1988) 533.
10. W. Keim, Green. Chem. 5 (2003) 105.
11. R. A. Sheldon, H.van Bekkum, Fine Chemicals through Heterogeneous Catalysis,
Wiley, Chichester, 2001.
12. S. M. George, Chem. Rev. 95 (1995) 475.
13. K. W. Kolansinski, Surface Science: Foundations of Catalysis and Nanoscience,
John Wiley and Sons Inc., West Chester, 2002.
14. G. Ertl, H. Knozinger, J. Weitkamp, Preparation of Solid Catalysts, Wiley-VCH,
Germany, 1999.
15. J. R. Anderson, Structure of metallic catalysts, Academic Press, London, 1975.
16. M. Haruta, T. Kobayashi, H. Sano, N. Yamada, Chem. Lett. 405 (1987) 405.
17. G. J. Hutchings, J. Catal. 96 (1985) 292.
18. L. Prati, A. Villa, Catal. 2 (2012) 24.
19. A. S. K. Hashmi, G. J. Hutchings, Angew. Chem., Int. Ed. 45 (2006) 7896.
20. S. A. K. Hashmi, Chem. Rev. 107 (2007) 3180.
21. A. Arcadi, S. Di Giuseppe, Curr. Org. Chem. 8 (2004) 795.
40
22. E. Jimenez-Nunez, A. M. Echavarren, Chem. Commun. (2007) 333.
23. G. C. Bond, C. Louis, D. T. Thompson, Catalysis by Gold, Imperial College Press,
London, 2007.
24. A. Corma, H. Garcia, Chem. Soc. Rev. 37 (2008) 2096.
25. G. H. Brundtland, Our common future: The World Commission on Environment
and Development, Oxford University Press, 1987.
26. A. Bruggink, R. Schoevaart, T. Kieboom, Org. Pro. Res. Dev. 7 (2003) 622.
27. J. C. Wasilke, S. J. Obrey, R. T. Baker, G. C. Bazan, Chem. Rev. 105 (2005) 1001.
28. D. E. Fogg, E. N. dos Santos, Coord. Chem. Rev. 248 (2004) 2365.
29. A. Kuhlmann, F. Roessner, W. Schwieger, O. Gravenhorst, T. Selvam, Catal. Today
97 (2004) 303.
30. M. J. F. M. Verhaak, A. J. Vandillen, J. W. Geus, Appl. Catal. A 109 (1994) 263.
31. A. Nissen, G. Heiien, E. Sapper, W. Fliege, A. Wittver, US Patent 4239657, 1980.
32. C. Carlini, M. Marchionna, M. Noviello, A. M. R. Galletti, G. Sbrana, F. Basile, A.
Vaccari, J. Mol. Catal. A 232 (2005) 13.
33. D. Tichit, B. Coq, S. Cerneaux, R. Durand, Catal. Today 75 (2002) 197.
34. P. Y. Chen, S. J. Chu, N. S. Chang, T. K. Chuang, L. Y. Chen, Stud. Surf. Sci.
Catal. 46 (1989) 231.
35. C. O. Veloso, J. L. F. Monteiro, E. F. Sousaaguiar, Stud. Surf. Sci. Catal. 84 (1994)
1913.
36. W. Nicol, E. L. du Toit, Chem. Eng. Proc. 43 (2004) 1539.
37. P. P. Yang, Y. C. Shang, J. F. Yu, J. Wang, M. L. Zhang, T. G. Wu, J. Mol. Catal. A
272 (2007) 75.
38. P. P. Yang, H. F. Yu, Z. L. Wang, M. P. Xu, Q. S. Liu, X. W. Yang, T. H. Wu,
Catal. Comm. 6 (2005) 107.
39. S. M. Yang, Y. M. Wu, Appl. Catal. A 192 (2000) 211.
40. E. F. Kozhevnikova, I. V. Kozhevnikov, J. Catal. 238 (2006) 286.
41. R. D. Hetterley, E. F. Kozhevnikova, I. V. Kozhevnikov, Chem. Commun. (2006)
782.
42. R. D. Hetterley, R. Mackey, J. T. A. Jones, Y. Z. Khimyak, A. M. Fogg, I.V.
Kozhevnikov, J. Catal. 258 (2008) 250.
43. F. Winter, A. J. van Dillen, K. P. de Jong, J. Mol. Catal. A 219 (2004) 273.
44. I. V. Kozhevnikov, Chem. Rev. 98 (1998) 171.
41
45. T. Okuhara, N. Mizuno, M. Misono, Adv. Catal. 41 (1996) 113.
46. I. V. Kozhevnikov, Catalysts for Fine Chemical Synthesis, Catalysis by
Polyoxometalates, Wiley, Chichester, 2002.
47. S. Liu, D. Volkmer, D. G. Kurth, J. Clust. Sci. 14 (2003) 405.
48. J. T. Rhule, C. L. Hill, D. A. Judd, Chem. Rev. 98 (1998) 327.
49. N. Mizuno, M. Misono, Chem. Rev. 98 (1998) 199.
50. M. Misono, Chem. Comm. (2001) 1141.
51. R. Wlodarczyk, M. Chojak, K. Miecznikowski, A. Kolary, P. J. Kulesza, R.
Marassi, J. Power Sources 159 (2006) 802.
52. F. Cavani, Catal. Today 41 (1998) 73.
53. M. J. Janik, R. J. Davis, M. Neurock, J. Phys. Chem. B 108 (2004) 12292.
54. J. Mater. Chem. A 2 (2014) 7637.
55. I. V. Kozhevnikov, J. Mol. Catal. A 262 (2007) 86.
56. U. B. Mioc, Ph. Colomban, M. Davidovic, J. Tomkinson, J. Mol. Struct. 326 (1994)
99.
57. J. B. Moffat, Metal-Oxygen Clusters: The Surface and Catalytic Properties of
Heteropoly Oxometalates, Kluwer, New York, 2001.
58. D. B. Taylor, J. B. Mcmonagle, J. B. Moffat, J. Colloid Interface Sci. 108 (1985)
278.
59. M. Misono, Catal. Rev. Sci. Eng. 30 (1988) 339.
60. T. Okuhara, T. Nishimura, H. Watanabe, M. Misono, J. Mol. Catal. 74 (1992) 247.
61. C. V. Hidalgo, H. Itoh, T. Hattori, M. Niwa, Y. Murakami, J. Catal. 85 (1984) 362.
62. Y. Izumi, R. Hasebe, K. Urabe, J. Catal. 84 (1983) 402.
63. F. Lefebvre, F. X. Liucai, A. Auroux, J. Mater. Chem. 4 (1994) 125.
64. A. Auroux, M. B. Sayed, J. C. Vedrine, Thermochim. Acta. 93 (1985) 557.
65. L. C. Josefowicz, H. G. Karge, E. Vasilyeva, J. B. Moffat, Microporous Mater. 1
(1993) 313.
66. I. V. Kozhevnikov, Russ. Chem. Rev. 56 (1987) 811.
67. Y. Izumi, K. Urabe, Chem. Lett. (1981) 663.
68. M. A. Schwegler, H. van Bekkum, Appl. Catal. 74 (1991) 191.
69. T. Baba, Y. Ono, Appl. Catal. 22 (1986) 321.
70. T. Matsuda, A. Igarashi, Y. Ogino, J. Jpn. Pet. Inst. 23 (1980) 30.
42
71. G. I. Kapustin, T. R. Brueva, A. L. Klyachko, M. N. Timofeeva, S. M. Kulikov, I.
V. Kozhevnikov, Kinet. Katal. 31 (1990) 1017.
72. G. I. Kapustin, T. R. Brueva, Thermochimica Acta. 379 (2001) 71.
73. B. B. Bardin, R. J. Davis, Appl. Catal. 200 (2000) 219.
74. T. Okuhara, Appl. CataL. A 256 (2003) 213.
75. G. Koyano, K. Ueno, M. Misono, Appl. Catal. A 181 (1999) 267.
76. S. Soled, S. Miseo, G. Mc Vicker, W. E. Gates, A Gutierrez, J. Paes, Catal. Today
36 (1997) 441.
77. S. Soled, S. Miseo, G. McVicker, W. E. Gates, A Gutierrez, J. Paes, Chem. Eng. J.
64 (1996) 247.
78. M. Misono, Mater. Chem. Phys. 17 (1987) 103.
79. T. Okuhara, T. Nakato, Catal. Surv. 2 (1998) 31.
80. Y. Izumi, K. Urabe, M. Onaka, Zeolite, Clay and Heteropoly Acid in Organic
Reactions, Kodansha/VCH, Tokyo, 1992.
81. T. Okuhara, Chem. Rev. 102 (2002) 3641.
82. A. Alhanash, E. F. Kozhevnikova, I. V. Kozhevnikov, Catal. Lett. 120 (2008) 307.
83. M. A. Alotaibi, E. F. Kozhevnikova, I. V. Kozhevnikov. App. Catal. 447– 448
(2012) 32.
84. K. A. D. Rocha, P. A. Robles-Dutenhefner, E. M. B. Sousa, E. F. Kozhevnikova, I.
V. Kozhevnikov, E. V. Gusevskaya, Appl. Catal. A 317 (2007) 171.
85. M. R. H. Siddiqui, S. Holmes, H. He, W. Smith, E. N. Coker, M. P. Atkins, I. V.
Kozhevnikov, Catal. Lett. 66 (2000) 53.
86. M. Musawir, E. F. Kozhevnikova, I. V. Kozhevnikov, J. Mol. Catal. A 262 (2007)
93.
87. G. W. Huber, S. Iborra, A. Corma, Chem. Rev. 106 (2006) 4044.
88. K. A. Hossain, Int. J. Renewable Energy Technol. Res.1 (2012) 23.
89. R. L. Lynd, J. H. Cushman, R. J. Nichols, C. E. Wyman, Science. 251 (1991) 1318.
90. C. E. Wyman, Appl. Biochem. Biotechnol. 897 (1994) 45.
91. D. Klemm, B. Heublein, H.-P. Fink, A. Bohn, Angew. Chem. 44 (2005) 2.
92. C. B. Field, M. J. Behrenfeld, J. T. Randerson, P. Falkowski, Science. 281 (1998)
237.
93. S. Tokgoz, A. Elobeid, An Analysis of the Link between Ethanol, Energy, and Crop
Markets, Ames, Iowa State University,2006.
43
94. L. Čuček, M. Martin, I. E. Grossmann, Z. Kravanja, Comp. Chem. Eng. 35 (2011)
1547.
95. D. Bacovsky, M. Dallos, M. Wörgetter, IEA Report 39 (2010) 1.
96. A. Corma, S. Iborra, A. Velty, Chem. Rev. 107 (2007) 2411.
97. E. L. Kunkes, D. A. Simonetti, R. M. West, J. C. Serrano-Ruiz, C. A. Gaertner, J. A.
Dumesic, Science 322 (2008) 417.
98. H. Bernas, K. Eranen, I. Simakova, A.-R. Leino, K. Kordas, J. Myllyoja, P. Maki-
Arvela, T. Salmi, D. Yu. Murzin, Fuel 89 (2010) 2033.
99. J. G. Immer, M. J. Kelly, H. H. Lamb, Appl. Catal. A 375 (2010) 134.
100. P. T. Do, M. Chiappero, L. L. Lobban, D. Resasco, Catal. Lett. 130 (2009) 9.
101. M. Arend, T. Nonnen, W. F. Hoelderich, J. Fischer, J. Groos, Appl. Catal. A 399
(2011) 198.
102. M. B. Smith, J. March, March’s Advanced Organic Chemistry: Reactions,
Mechanisms, and Structure, 6th ed., John Wiley & Sons, Inc., Hodoken, New
Jersey, 2007.
103. M. A. Alotaibi, E. F. Kozhevnikova, I. V. Kozhevnikov, Chem. Commun. 48 (2012)
7194.
104. M. A. Alotaibi, E. F. Kozhevnikova, I. V. Kozhevnikov, J. Catal. 293 (2012) 141.
105. S. Itagaki, N. Matsuhashi, K. Taniguchi, K. Yamaguchi, N. Mizuno, Chem. Lett.43
(2014) 1086.
106. E. Furimsky, Appl. Catal. A 199 (2000) 147.
107. Q. Bu, H. Lei , A. H. Zacher, L.Wang, S. Ren, J. Liang, Y. Wei, Y. Liu, J. Tang, Q.
Zhang, R. Ruan, Bioresour.Technol. 124 (2012) 470.
108. J. Wildschut, F. H. Mahfud, R. H. Venderbosch, H. J. Heeres, Ind. Eng. Chem. Res.
48 (2009) 10324.
109. T. T. Isimjan, Q. He, Y. Liu, J. Zhu, R. J. Puddephatt, D. J. Anderson, ACS
Sustainable Chem. Eng. 1 (2013) 381.
110. R. A. Augustine, Heterogeneous Catalysis for the Synthetic Chemist, Marcel
Dekker, New York, Inc., N. Y., 1996.
111. A. D. Murkute, J. E. Jackson, D. J. Miller, J. Catal. 278 (2011) 189.
112. M. Renz, European Journal of Organic Chemistry (2005) 979.
113. T. Yokoyama, N. Yamagata, Appl. Catal. A 221 (2001) 227.
44
114. H. Benaissa, P. N. Davey, E. F. Kozhevnikova, I. V. Kozhevnikov, Appl. Catal. A
351 (2008) 88.
115. H. Benaissa, P. N. Davey, Y. Z. Khimyak, I. V. Kozhevnikov, J. Catal. 253 (2008)
244.
116. H. -J. A. K. Weissermel, Industrial Organic Chemistry, 3rd ed., VCH, Weinheim,
1997.
117. J. J. Gamman, S. D. Jackson, F. A. Wigzell, Ind. Eng. Chem. Res. 49 (2010) 8439.
118. G. Waters, O. Richter, B. Kraushaar-Czarnetzki, Ind. Eng. Chem. Res. 45 (2006)
6111.
119. G. Waters, O. Richter, B. Kraushaar-Czarnetzki, Ind. Eng. Chem. Res. 45 (2006)
5701.
120. F. Zaccheria, N. Ravasio, M. Ercoli, P. Allegrini, Tetrahedron Lett. 46 (2005) 7743.
121. Y.Jiang, R.Buchel, J.Huang, F.Krumeich, S.E.Pratsinis, A.Baiker, ChemSusChem.
5 (2012) 1190.
122. B. G. R. H. A. Wittcoff, Industrial Organic Chemicals, Wiley, N. Y., 1996.
123. F. P. Heese, M. E. Dry, K. P. Moller, Catal. Today 49 (1999) 327-335
124. T. L. Marker, H. U. Hammershaimb, R. Marinangeli, W. H. Keesom, U. G.
Bozzano, AIChE Annual Meeting, 1993.
125. T. L. Marker, W. H. Keesom, R. Schmidt, S. Davis, R. Marinangeli, Conference on
Clean Air Act and Reformulated Gasolines, 1994.
126. T. L. Marker, G. A. Funk, H. U. Hammershaimb, US Patent 5504258, 1996.
127. R. J. Taylor, Jr., P. E. Dai, J. F. Knifton, US Patent 5583266, 1996.
128. H. Y. Zhao, D. Li, P. Bui, S. T. Oyama, Appl. Catal. A 391 (2011) 305.
129. C. Zhao, J. A. Lercher, ChemCatChem 4 (2012) 64−68.
130. X. L. Zhu, L. L. Lobban, R. G. Mallinson, D. E. Resasco, J. Catal. 281 (2011) 21.
131. C. Zhao, Y. Kou, A. A. Lemonidou, X. Li, J. A. Lercher, Chem. Commun. 46
(2010) 412.
132. C. R. Lee, J. S. Yoon, Y. W. Suh, J. W. Choi, J. M. Ha, D. J. Suh, Y. K. Park,
Catal. Commun. 17 (2012) 54.
133. C. Zhao, J. He, A. A. Lemonidou, X. Li, J. A. Lercher, J. Catal. 280 (2011) 8.
134. Y. Yang, C. Ochoa-Hernández, V. A. de la Pe˜na O’Shea, P. Pizarro, J. M.
Coronado, D. P. Serrano, Appl. Catal. B 145 (2014) 91.
45
135. D. Y. Hong, S. J. Miller, P. K. Agrawala, C. W. Jones, Chem. Commun. 46 (2010)
1038.
136. X. Zhu, L. Nie, L. L. Lobban, R. G. Mallinson, D. E. Resasco, Energy Fuels 28
(2014) 4104.
137. S. S. Kanwar, H. K. Verma, R. K. Kaushal, R. Gupta, S. S. Chimni, Y. Kumar, G. S.
Chauhan, World J. Microbiol Biotechnol, 21 (2005) 1037.
138. R. A. Y. Jones, Physical and Mechanistic Organic Chemistry, Cambridge University
Press, Cambridge, 1979, 227.
139. P. L. Silva, C. M. Silva, L. Guimarães, J. R. Pliego Jr., Theor Chem Acc 134 (2015)
1591.
140. Y. Zhou, H. Fu, X. Zheng, R. Li, H. Chen, X. Li, Catal. Comm. 11 (2009) 137.
141. O. I. Senol, T. R. Viljava, A. O. I. Krause, Catal. Today 106 (2005) 186.
142. B. Zhang, L. Lin , J. Zhuang, Y. Liu, L. Peng, L. Jiang, Molecules 15 (2010) 5139.
46
2. Experimental
2.1 Introduction
This chapter will describe the experimental techniques that were used in this study. It will begin
with the synthesis of the metal-HPA bifunctional catalysts, before moving on to explore the
catalyst characterisation techniques used to determine the catalyst’s surface and porosity
properties, element content, particle sizes, catalyst stability and metal dispersion. Finally, the
experimental set up of the reaction studies will be described, in addition to the calculations of the
conversion and the product selectivity.
2.2 Materials
MIBK (99%), diisobutyl ketone (80%), acetophenone (≥98%), 2-octanone (98%),
cyclohexanone (≥99%), 3-pentanone (≥99%), diisopropyl ether (≥98.5%), ethyl propanoate
(99%) and inorganic chemicals used for catalysts preparation were purchased from Aldrich, and
anisole (99%) was from Avocado, and 2-butanone (99%) and 2-hexanone (98%) from Acros
Organics. Heteropoly acid hydrates, H3PW12O40 (HPW, 99%) and H4SiW12O40 (HSiW, 99.9%)
containing 20-28 H2O molecules per Keggin unit, were purchased from Sigma-Aldrich. The
amount of crystallization water in the HPAs was determined by TGA. Carbon-supported
platinum 10%Pt/C (7.1% Pt content in dried catalyst from ICP analysis) was from Johnson
Matthey. H2 and N2 gases (>99%) were supplied by the British Oxygen Company.
Catalyst supports Aerosil 300 silica (surface area SBET = 300 m2g-1) and P25 titania (anatase/rutile
= 3:1, SBET = 44 m2g-1) were from Degussa.
47
2.3 Catalyst preparation
2.3.1 Preparation of CsnH3-nPW12O40
Cs2.5H0.5PW12O40 (CsPW) and Cs2.25H0.75PW12O40 were prepared according to the literature
procedure [1] by adding dropwise the required amount of aqueous solution of Cs2CO3 (0.47 M)
to an aqueous solution of H3PW12O40 (0.75 M) at 40 oC with continuous stirring. The precipitates
obtained were aged in aqueous slurry for 24 h at room temperature. The slurry was then slowly
evaporated to dryness in a rotary evaporator at 45 oC to afford the catalysts as white powder. The
catalysts were calcined under vacuum at 150 oC/10-3 kPa for 1.5 h and ground to 45-180 μm
particle size.
2.3.2 Preparation of Pt, Ru, Cu, Ni and Au modified CsPW
Bifunctional metal-acid catalysts were prepared by wet impregnation of CsPW with an
appropriate metal precursor (Pt(acac)2, H2PtCl6, RuCl3, Ni(NO3)2, Cu(NO3)2 and HAuCl4)
followed by reduction of metal ion to metal with H2. The metal loadings quoted were confirmed
by the ICP-AES elemental analysis; these were in good agreement with the preparation
stoichiometries since the preparations did not involve operations such as filtration and washing
which could cause metal loss.
2.3.2.1 Preparation of Pt/CsPW
0.5%Pt/CsPW was prepared by stirring CsPW powder with 0.02 M Pt(acac)2 solution in benzene
at room temperature for 1 h, followed by slow evaporation of benzene in a rotary evaporator at
room temperature [2]. The catalyst was calcined under vacuum at 150 oC/10-3 kPa and then
reduced in an oven by a hydrogen flow at 250 oC for 2 h. Two other modifications of this catalyst
were prepared by impregnation of CsPW with an aqueous solution of H2PtCl6, followed by
drying in a rotary evaporator at 45 oC and the same calcination and reduction procedures. One,
48
designated as 0.5%Pt/CsPW-I, was prepared by direct wet impregnation of CsPW powder with
0.1 M aqueous solution of H2PtCl6 involving stirring the aqueous slurry for 24 h at room
temperature, followed by the workup procedure. The other, designated as 0.5%Pt/CsPW-A, was
prepared by adding 0.1 M aqueous solution of H2PtCl6 to the freshly precipitated aqueous CsPW
slurry and ageing the mixture at room temperature with stirring for 24 h, followed by the workup.
For comparison with Au/CsPW and Pt/Au/CsPW catalysts, different Pt loadings of Pt/CsPW-I
catalyst were used (0.3-6 wt%).
The physical mixture of 7%Pt/C and CsPW containing 0.35% of Pt was prepared by grinding a
1:19 w/w mixture of the two components.
10%Pt/SiO2 was prepared by impregnation of Aerosil 300 silica with Pt(acac)2 from benzene,
followed by reduction with H2 at 250 oC for 2 h. The physical mixture 10%Pt/SiO2 + CsPW
containing 0.5% of Pt was prepared by grinding a 1:19 w/w mixture of the two components.
2.3.2.2 Preparation of Ru/CsPW
Two modifications of 5%Ru/CsPW, designated as 5%Ru/CsPW-I and 5%Ru/CsPW-A, were
prepared by impregnation of CsPW with 0.1 M aqueous solution of RuCl3 similar to the
preparation of 0.5%Pt/CsPW-I and 0.5%Pt/CsPW-A.
2.3.2.3 Preparation of Cu/CsPW
10%Cu/CsPW catalyst, designated as 10%Cu/CsPW-I, was prepared as described elsewhere [2]
by stirring CsPW powder with an aqueous solution of Cu(NO3)2.6H2O for 24 h at room
temperature, followed by drying in a rotary evaporator at 65 oC and calcination at 150 oC/10-3
kPa for 1.5 h. Finally, the sample was reduced in H2 flow at 400 oC for 2 h. Another modification
of this catalyst, designated as 10%Cu/CsPW-A, was prepared by adding an aqueous solution of
49
Cu(NO3)2∙6H2O to the freshly precipitated aqueous CsPW slurry and ageing the mixture at room
temperature with stirring for 24 h, followed by the same workup.
2.3.2.4 Preparation of Ni/CsPW
Two modifications of 10%Ni/CsPW catalyst, designated as 10%Ni/CsPW-I and 10%Ni/CsPW-
A, were prepared similarly to the corresponding 10%Cu/CsPW catalysts using Ni(NO3)2∙6H2O
as a precursor.
2.3.2.5 Preparation of Au/CsPW
Au/CsPW was prepared by wet impregnation of CsPW powder with aqueous solutions of
HAuCl4. This involved stirring the aqueous slurry at 50 oC for 2 h followed by rotary evaporation
to dryness and reduction with H2 flow at 250 oC for 2 h. This catalyst had metal loadings between
0.3 – 6 wt%.
2.3.2.6 Preparation of bimetallic Pt/Au/CsPW catalysts
The bimetallic PtAu/CsPW catalysts were prepared similarly; PtAu/CsPW-CI was prepared by
co-impregnation, whereas PtAu/CsPW-SI by sequential impregnation of CsPW with H2PtCl6 and
HAuCl4. The sequential procedure included preparation of Pt/CsPW with reduction by H2 at 250
oC/2 h followed by impregnation of the Pt/CsPW thus made with HAuCl4 aqueous solution and
subsequent reduction (H2/250 oC/2 h). In this case, the Pt was treated twice with H2 at 250 oC.
Physically mixed metal-acid bifunctional catalysts 5%Pt/C + CsPW, 5%Au/C + CsPW,
5%Pt/5%Au/C + CsPW, and 5%Pt/10%Au/C + CsPW with 0.5% Pt loading were prepared by
grinding a 1:9 w/w mixture of the corresponding two components. 5%Pt/C, 5%Au/C and PtAu/C
catalysts were prepared in-house (see below).
50
2.3.3 Preparation of carbon-supported metal catalysts
Carbon-supported 5%Pt/C and 5%Au/C catalysts were prepared by wet impregnation of Darco
KB-B activated carbon with aqueous solutions of H2PtCl6 and HAuCl4 at 50 oC for 2 h followed
by rotary evaporation to dryness and reduction by H2 flow as above.
Bimetallic PtAu/C catalysts were prepared similarly by wet co-impregnation of the Darco KB-B
carbon with H2PtCl6 and HAuCl4 (Pt/Au = 1:1 and 1:2 mol/mol); these are hereafter referred to
as 5%Pt/5%Au/C-CI and 5%Pt/10%Au/C-CI, respectively. PtAu/C catalysts were also prepared
by sequential impregnation either by wet-impregnating the pre-made 5%Pt/C with the required
amount of HAuCl4 followed by reduction with H2 at 250 oC/2 h (referred to as 5%Pt/5%Au/C-SI
and 5%Pt/10%Au/C-SI) or the other way around by wet-impregnating the pre-made 5%Au/C
with H2PtCl6.
2.3.4 Preparation of supported hetropoly acid catalysts
Catalyst supports Aerosil 300 silica (surface area SBET = 300 m2g-1) and P25 titania (anatase/rutile
= 3:1, SBET = 44 m2g-1) were from Degussa. ZrO2 (SBET = 107 m2g-1) and Nb2O5 (SBET = 187 m2g-
1) were prepared in-house (see below) [3] and calcined at 400 oC in air for 5 h.
Supported 15 wt% HPA catalysts were prepared by wet impregnation of the oxide supports with
an aqueous HPA solution [3-5]. An oxide support (8.5 g) was mixed with the required amount
of aqueous HPW solution. Then the slurry formed was left to age with stirring for 24 h at room
temperature. After that, the catalyst was dried in a rotary evaporator. The catalyst was calcined
in air for 3 h at a temperature ranged from 100 to 500°C.
15%HPW/SiO2 catalyst was prepared by wet impregnation [6]. A suspension of 8.5 g Aerosil
300 silica (300 m2/g) in 60-80 ml aqueous solution, containing a certain amount of heteropoly
51
acid, was stirred overnight at room temperature. After that, the catalyst was dried in a rotary
evaporator. Then the catalyst was calcined under vacuum at 150oC/0.1 kPa for 1.5 h.
Supported hetropoly acid catalysts were kindly provided by Mrs W. Alharbi.
2.3.5 Preparation of Nb2O5
Nb2O5 (SBET = 187 m2g-1) was made by the literature method [7]. It was prepared by dissolving
a certain amount of NbCl5 powder in ethanol and adding to aqueous solution of ammonia
hydroxide (0.3 M) to form a white precipitate of Nb2O5.nH2O. The precipitate was filtered off
and washed with distilled water many times until chloride free, as tested with AgNO3 (Equation
2.1). Finally, the niobic acid was left overnight in an oven for drying at 100 oC.
AgNO3 (aq) + Cl- (aq) AgCl (s) + NO3- (aq) (2.1)
2.3.6 Preparation of ZrO2
ZrO2 was prepared according to the literature procedure [3]. At room temperature, aqueous
ammonium hydroxide (30%) was added dropwise into an aqueous solution of ZrOCl2 (5 g of
ZrOCl2 in 42 ml of water) with intense stirring until the pH 10 was reached. After that, the
hydrogel produced was left with stirring at room temperature for 24 h and then filtered through
a Buchner funnel. Distilled water was used to wash the white precipitate until chloride free, as
tested with AgNO3. Finally, the white precipitate was dried in an oven for 24 h at 100 oC.
52
2.4 Catalyst characterisation techniques
2.4.1 Surface area and porosity analysis
Generally, heterogeneous catalysts are porous materials. The surface area and pore texture of
these materials can have significant effect on their activity, selectivity and stability. According
to the IUPAC classification, pore sizes are classified into three size groups [8]:
1. Micropores – size < 2 nm, ultramicropores size < 0.7 nm
2. Mesopores – 2 nm < size < 50 nm
3. Macropores – size > 50 nm.
Porous solids have much higher total surface area than the external surface area as a result of the
contribution of the porous cavity walls. In general, the total surface area of heterogeneous
catalysts is between 1 and 1000 m2 per gram, and from 0.1-10 m2 per gram for the external surface
area [8].
Nitrogen adsorption at boiling temperature -196 oC (77 K) is a very common technique to
measure the catalyst surface area and its porous texture [8-11].
A well-defined procedure for determination of the total surface area of porous materials is the
Brunauer-Emmett-Teller (BET) method, developed in 1938 [8, 12]. The BET surface area is
measured from the BET plot using the relative pressure (P/PO) usually in the range between 0.05
and 0.35. From the BET isotherm, the monolayer volume of adsorbed nitrogen gas, Vm, and the
solid surface area, As, are calculated using equation 2.2 and 2.3, respectively:
P
V(P0 − P)=
1
VmC+
C − 1
VmC
P
P0 (2.2)
In this equation, P is the pressure of adsorbate gas at equilibrium with the surface, Po is the
saturation pressure, V is the adsorbed gas volum and C is the BET constant.
53
P/V(PO – P) is plotted against the relative pressure (P/PO) according to equation 2.2; this plot
should give a straight line with the slope (C-1)/VmC and the intercept 1/VmC.
Finally the surface area can be calculated from equation 2.3.
As = (Vm / 22414)Naσ (2.3)
In this equation, Na is the Avogadro number (6.022 1023 mol-1) and σ is the area covered by one
nitrogen molecule, 0.162 nm2 [10].
The BET surface area and porosity of catalysts were determined from nitrogen physisorption
measured on a Micromeritics ASAP 2010 instrument at −196 oC. Before measurement, the
samples (typically 0.2 g) were evacuated at 250 oC for 2 h. After degassing, the sample was
allowed to cool to room temperature and reweighted to adjust the sample weight. Then, the
sample tube was dipped in liquid nitrogen. Finally, the gas pressure was allowed to reach
equilibrium before subsequent dosing and then a series of 55 successive nitrogen doses were
applied in order to gain an adsorption isotherm. The surface analyser used is shown in Figure
2.1.
Figure 2.1 Micromeritics ASAP 2010 analyser utilised to determine the surface area and porosity
of catalysts [13].
54
2.4.2 Inductively coupled plasma atomic emission spectroscopy (ICP-AEC)
ICP-AEC is an important technique for elemental analysis of catalyst samples. Solid samples are
dissolved in a liquid, usually an acidic solution. Then, a plasma source (a gas mixture of positive
ions and electrons created by heating gases such as argon at ≈6000 K) is used to ionize molecules
and excite them to higher energy levels. Once they have returned to the ground state, they emit
light with characteristic emission lines [14]. The intensity of the light can tell how much of each
element is contained within the sample, whereas the wavelengths tell which elements are present.
The spectral intensity of elements is proportional to the concentration of these elements in the
sample, so the intensity produced can be compared to a standard curve to calculate the
concentrations of the elements in the sample.
In this study, ICP spectroscopy was used to quantify metal content in the doped CsPW and active
carbon. This experiment was kindly performed on a Spectro Ciros emission spectrometer by G.
Miller at Liverpool University in Chemistry Department.
2.4.3 Powder X-ray diffraction (XRD)
XRD diffraction is one of the most important characterization technique used to study the phase
structure of solid materials. The wavelengths of X-rays are equivalent to the atom spacing in
crystals, so they are able to go through these materials, resulting in characteristic diffraction
patterns. There are different factors that affect the scattering angle of X-rays which obeys the
Bragg,s law (Equation 2.4) [15].
nλ = 2dSinθ (2.4)
In Equation 2.4, n is the reflection order (an integer value), λ is the X-ray wavelength, d is the
lattice planar spacing, θ is the angle of diffraction.
55
The X-ray diffractogram can provide valuable information including the crystallinity of solid
materials, the dimensions and symmetries of the unit cell and the average particle size, which
can be determined from the Scherrer equation (2.5):
t = 0.9λ/B cosθ (2.5)
Here t is the particle thickness, λ is the incident X-ray wavelength, B is the full-width at half
maximum of the diffraction peak and θ is the diffraction angle. In this work, Pt, Au and Cu metal
particle size was measured using this method [16].
In this work, powder X-ray diffraction (XRD) of catalysts were recorded on a PANalytical Xpert
diffractometer with a CuKα radiation (λ= 1.542 Å). XRD patterns were attributed using the
JCPDS database.
2.4.4 H2 chemisorption
It is important to be able to determine the metal dispersion on the surface of the catalyst support
in order to understand the activity of supported metal catalysts. The catalyst activity usually
increases as the metal dispersion increases. For practical reasons, gas adsorption techniques are
commonly used to determine the metal dispersion [17]. Metal dispersion is defined as the ratio
of the total number of metal atoms which are at the surface of the metal particles to the total
number of metal atoms in the catalyst. The metal particle size can be calculated from the
dispersion of the metal atoms [18].
The metal dispersion of Pt and Ru in our catalysts was measured in a flow system by hydrogen
chemisorption using the hydrogen-oxygen titration pulse method, which is a more sensitive and
convenient analytic method to determine metal dispersion in supported metal catalysts [19, 20].
This technique has previously been used for dispersion measurement of Pd [20], Pt [19], Ru
[21] and Rh [22].
In this study M/CsPW catalysts were reduced in hydrogen flow at 250 °C for 2 h to convert
56
metal precursor to M0. A catalyst sample (50 mg) reduced by hydrogen was pre-exposed to air
at room temperature for 1 h to allow O2 to adsorb onto the metal atoms on the catalyst surface
(Ms) at an Ms/O ratio of 1:1. Then the sample was placed in a glass sample tube connected
to a Micromeritics TPD/TPR 2900 instrument equipped with a thermal conductivity detector
(TCD) and stabilised at a specified temperature under nitrogen flow (Figure 2.2). The
hydrogen-oxygen titration was carried out at room temperature for Pt catalysts and at 100 oC
for Ru catalysts. 20 µl pulses of pure H2 (heated to 75°C) were injected in the N2 flow in 3 min
intervals until the catalyst was saturated with hydrogen. The metal dispersion, D, defined as the
fraction of metal (M) at the surface, D = Ms/Mtotal, was calculated assuming the stoichiometry
of H2 adsorption (Equation 2.5) [19, 20]:
MsO + 1.5 H2 → MsH + H2O (2.6)
Figure 2.2 Micromeritics TPD/TPR 2900 analyzer used for conducting H2 chemisorption
experiments.
57
The hydrogen volume adsorbed onto the surface of the catalyst was determined through
integrating areas under peaks displayed on the screen which were detected by TCD. Pulses were
repeated until no more adsorption was occurred, then the total volume of hydrogen adsorbed at
75 ºC (348 K) was calculated. Equations 2.7-2.10 were used to calculate the dispersion and
average M particle diameter [23].
V348K (μl) = Σ {20 – [(PAads/PAav) x 20] (2.7)
In equation 2.7, V348K is the total volume adsorbed of H2 (µL) at 75 ºC (348 K), PAads is the peak
area of adsorbed H2, PAav is the mean average peak area of blank H2 injections in the absence of
catalyst.
V273 K (μl) = V348K × (273/348) (2.8)
In equation 2.8, V273 K is the total volume (µL) of adsorbed H2 at 0 ºC (273 K).
D =V273(ml)× Ar(g/ mol)
Mcat(g)×22414(ml/mol)×CM ×1.5 (2.9)
In equation 2.9, D is the metal dispersion, Ar is the relative atomic mass of M, mcat is the mass of
catalyst used (g), 22414 represents the volume of one mole of H2 gas at 0 ºC (273 K), CM is the
concentration of M as a fraction of the catalyst mass, and 1.5 is the stoichiometry of H2 adsorbed
onto M.
The average diameter of metal particles, d, was obtained from the empirical equation 2.10 [20].
d (nm) = 0.9/D (2.10)
2.4.5 CO chemisorption
In this study Pt dispersion for the commercial 7%Pt/C catalyst was determined by pulse
chemisorption of CO on a Micromeritics TPD/TPR 2900 apparatus at 50 oC in He flow (20 mg
catalyst sample, 50 μL pulses of pure CO, adsorption stoichiometry Pts:CO = 1).
58
Figure 2.3 CO pulse adsorption on the surface of 7%Pt/C, with CO signals detected
during the pulsation.
2.4.6 Thermogravimetric analysis (TGA)
Thermogravimetric analysis (TGA) is commonly used to investigate the changes that accompany
the programmed heating of the sample. The change in the mass of the material may refer to
chemical or physical changes, which can be determined as percentage value. A TGA instrument
consists of a sensitive balance comprising a pan loaded with the sample, which is then placed
inside a programmed furnace (Figure 2.4). TGA curves display the change in the weight in
relation to the changes in temperature. This weight loss curve provides information about
changes in the sample composition, thermal stability and kinetic parameters for the chemical
reactions in the sample. A derivative thermogravimetric (DTG) weight loss curve can be used to
show the point at which weight loss is most apparent [24].
In this study, a Perkin Elmer TGA 7 instrument was used to conduct the thermogravimetric
experiments. In these experiments, the temperature was increased from room temperature to 700
Injection time (min)
Sign
al m
V/1
00
00
59
ºC with a rate of heating of either 10 or 20 ºC/ min, under continuous N2 flow. Figure 2.5 shows
an example of TGA for HPW hydrate.
Figure 2.4 Diagram of TGA instrument [25].
Figure 2.5 TG/DTG for HPW hydrate.
88
90
92
94
96
98
100
102
0 100 200 300 400 500 600 700 800
We
igh
t %
Temperature ◦C
60
2.4.7 Elemental analysis
In this study carbon and hydrogen content in spent catalysts was determined with the aim of
studying the influence of coke on catalyst performance. This determination was carried out using
combustion analysis, which was performed on a Thermo Flash EA 1112 series analyser in the
Chemistry Department at Liverpool University.
2.4.8 Microcalorimetry
Calvet calorimeters proved to be very valuable tools for the measurement of heats of chemical
reactions. In this work, a Setaram C80 heat flux Calvet type microcalorimeter was utilized to
measure the heat of ammonia adsorption by solid HPA catalysts. The setup includes two vessels,
one for the sample and one for the reference, that are placed in the calorimetric block (Figure
2.6), which functions as a heat sink.
Figure 2.6 Setaram C80 calorimeter gas vessel [26].
61
Each catalyst sample (0.5-1 g) was pre-treated at 150 oC in a dry nitrogen atmosphere (20
mL/min) for 90 minutes. Once equilibrium attained, the experiment was initiated by successive
pulses of gaseous ammonia (0.5 mL, 0.02 mmol) into the N2 flow using a stainless steel loop
fitted in a 10 port Valco valve, and ammonia was injected every 30 minutes. The amount of
ammonia adsorbed onto the catalyst was calculated by difference between the initial amount of
ammonia and the amount of ammonia broken through the sample cell.
Ammonia adsorption measurement was kindly performed by Mrs W. Alharbi.
2.4.9 Scanning transmission electron microscopy (STEM) with energy
dispersive X-ray emission (EDX) microanalysis
STEM-EDX was used in this study to analyse supported Pt, Au and PtAu catalysts: Pt/CsPW,
Au/CsPW and PtAu/CsPW.
The STEM-EDX measurements were kindly performed by Dr D. Belic.
2.4.9.1 STEM
A focussed beam of electrons is tunnelled between the tip of a probe and the surface of the
sample, generating an electrical signal. The electron probe can be scanned over the sample,
allowing a computer-generated image to be created of the sample surface in a raster pattern.
STEM experiments are carried out under a high vacuum to reinforce the signal. The mean surface
particle diameter, dsp, is defined as Σnidi3/ Σnidi
2, where ni is the number of metal particles of a
dimeter di [27].
2.4.9.2 EDX
To develop a map of the surface, an energy dispersive X-ray emission detector (EDX) is placed
close to the sample grid in STEM. EDX is used to identify and assess the particular elements and
their relative proportions in STEM images. When the STEM electron beam passes throw the
sample, electrons from lower energy “inner” electron shells may be excited and ejected from that
62
energy state, leaving holes. These holes are filled by electrons from higher energy “outer” shells,
and in doing so, emit X-rays with an energy that is representative of the difference in energy
between the two electron shells where the transition occurs. The X-ray emission lines produced
are characteristic of the elements contained within the sample [27].
STEM imaging and EDX analysis of catalysts was carried out on an aberration-corrected JEOL
JEM 2100FCs instrument operated at 200 kV, equipped with an EDAX Octane T Optima 60
windowless silicon drift detector. For STEM analysis, the samples were prepared by scooping
up the powder catalyst by a TEM grid (holey carbon film on 300 Ni mesh, Agar Scientific)
followed by shaking to remove excess material from the grid.
2.4.10 Fourier transform infrared spectroscopy (FTIR).
Infrared spectroscopy is a widely used technique for the determination of solid catalyst structures
[12, 15, 28, 29]. Infrared radiation, approximately in the region 400-4000 cm-1 can be used to
investigate the fundamental vibration of sample chemical bonds. The chemical bonds absorb the
infrared radiation, bending and stretching them. The frequency of the radiation required to excite
the vibrational modes, and the intensity of absorption, depends on the strength and chemical
environment of the bonds. The infrared radiation scans the sample and when absorption occurs,
the transmitted infrared beam is weakened. The resulting spectrum (the intensity is plotted
against wave number (λ)) can be presented in either absorption or transmission mode. In this
technique, diffusely scattered light can be directly collected from the catalyst with a mirror which
is then passed to a detector (Figure 2.7). This method can be applied for sampling catalyst
powders.
Fourier transform infrared spectroscopy of adsorbed pyridine has been widely used to study
Lewis and Bronsted acid sites on the catalyst surface [3, 30-32].
63
Figure 2.7 Schematic of the diffuse reflectance accessory [33].
A Nicolet NEXUS FTIR spectrometer was used in this study to confirm the stability of HPA
catalysts and to examine the presence of Lewis acid and Bronsted sites by adsorption of pyridine.
The investigation of the Keggin structure for fresh and spent HPA catalysts was carried out in
the range of 500-1200 cm-1. The investigation of Lewis and Bronsted acid sites was performed
in the region of 1450 and 1540 cm-1, respectively. Catalyst preparation involved degassing of
samples under vacuum for 1.5 h at 150 °C, then careful grinding to make a diffusely scattering
matrix by mixing 0.025 g of catalysts with 0.475 g of potassium bromide (20 wt %). Such matrix
results in lower absorption and thus greater beam throughput, enhancing analysis resolution.
2.5 Catalytic reaction studies
2.5.1 Hydrodeoxygenation of biomass-derived ketones
The hydrogenation of ketones was carried out in the gas phase in flowing H2. The catalysts were
tested at 60-100 oC under atmospheric pressure in a Pyrex fixed-bed down-flow reactor (9 mm
internal diameter) fitted with an on-line gas chromatograph (Varian Star 3400 CX instrument
with a 30 m x 0.25 mm HP INNOWAX capillary column (column A) and a flame ionisation
64
detector). For more accurate hydrocarbon analysis, a 60 m x 0.32 mm GSGasPro capillary
column (column B) was utilised which provides better separation for C1-C3 hydrocarbons. The
temperature in the reactor was controlled by a Eurotherm controller using a thermocouple placed
at the top of the catalyst bed. The gas feed contained a variable amount of ketone in H2 as a
carrier gas. The ketone was fed by passing H2 carrier gas flow controlled by a Brooks mass flow
controller through a stainless steel saturator, which held the liquid ketone at appropriate
temperature to maintain the chosen reactant partial pressure. The downstream gas lines and
valves were heated to 180 oC to prevent substrate and product condensation. The gas feed entered
the reactor at the top at a flow rate of 20-100 mL min-1. The reactor was packed with 0.2 g catalyst
powder of 45-180 μm particle size. In some cases, to reduce conversion a smaller amount of
catalyst was used as a homogeneous mixture with silica of a total weight of 0.2 g. Prior to
reaction, the catalysts were pre-treated in H2 for 1 h at the reaction temperature unless stated
otherwise. The dehydration of 2-methyl-4-pentanol was studied similarly, except using N2 as a
carrier gas instead of H2. Once reaction started, the downstream gas flow was analysed by the
on-line GC to obtain reactant conversion and product selectivity. Reactant conversion (X),
product yields (Yp) and product selectivities (Sp,s) were calculated using equations (2.11-2.13).
The mean absolute percentage error in conversion and selectivity was ≤ 10% and the carbon
balance was maintained within 95%.
Yp =Sp×Kg×A
Sr+(∑ Sp×Kg×A)×100 (2.11)
X = ∑ 𝑌𝑝 (2.12)
𝑆𝑝,𝑠 =𝑌𝑝
X×100 (2.13)
In equations (2.11-2.13): Sr is the area count of unreacted substrate, Sp is the product peak area,
Kg is the calibration factor of product relative to the substrate, A is the product stoichiometry
factor relative to the substrate and (ΣSP x Kg x A) is the summation for all products in reaction.
65
Figure 2.8 Equipment setup for gas-phase reaction, where (a) 3-way valve, (b) check valve
(non return).
The activation energy of MIBK conversion over Pt/CsPW catalysts was measured using the
Arrhenius equation (2.14) at the temperature between 80-110°C under differential conditions
within the conversion range < 10%.
𝑘 = 𝐴e−EaRT (2.14)
Here k is the rate constant of the reaction, A is the pre-exponential factor, Ea is the activation
energy, R is the universal gas constant and T is the absolute temperature in Kelvin. Ea can be
determined from the straight line gained from a plot of ln k against 1/T using following equation:
ln k = lnA −Ea
RT (2.15)
66
2.5.2 Deoxygenation of ethers and esters
Deoxygenation (decomposition) of anisole, diisopropyl ether (DPE) and ethyl propanoate (EP)
ester was carried out in the gas phase in flowing H2 or N2. The catalysts were tested under
atmospheric pressure in the same reactor which was used in hydrodeoxygenation of ketones. The
substrates were fed by passing the carrier gas flow controlled by a Brooks mass flow controller
through a stainless steel saturator, which held the liquid substrate at appropriate temperature (±1
oC) to maintain the chosen reactant partial pressure. The downstream gas lines and valves were
heated to 150 oC to prevent substrate and product condensation. The gas feed entered the reactor
at the top at a flow rate of 20 mL min-1. The reactor was packed with 0.20 g catalyst powder of
45-180 μm particle size. Prior to reaction, the catalysts were pre-treated in situ for 1 h at the
reaction temperature. Reactant conversion, product selectivity and activation energy were
calculated using equations 2.11-2.15.
2.6 Product analysis
2.6.1 Gas chromatography
Gas chromatography is very common technique used in analytical chemistry to separate mixtures
of volatile compounds. The mobile phase is a carrier gas, such as helium, argon, hydrogen or
nitrogen, which mixes with the volatile compounds and is then passed through a column
containing a solid or liquid stationary phase [15]. This stationary phase separates the mixture
passing through it, with the products progressing through to the detector at different times
depending on their boiling points and solubility.
Although different types of detector can be utilised, the flame ionization detector (FID) is often
used for analysing organic compounds (Figure 2.9). Within the FID detector, the effluent from
the column is heated with a high temperature flame created by mixing H2 and air, which then
67
ionises solute molecules having low ionisation potential. The amount of charge formed is
proportional to the concentration of ions derived from the solutes. The current is then amplified
using the built-in computer and a chromatogram is produced on an external computer. A
schematic diagram of a gas chromatograph is provided in Figure 2.10.
Figure 2.9 Flame ionization detector (FID) [34].
Figure 2.10 Diagrammatical representation of gas chromatograph set up [34].
68
2.6.2 GC calibration
In this study, all components in a mixture of volatile compounds were quantified using the
internal standard method. Calibration factors were determined by preparing a series of solutions
with a different concentration of analyte and a constant concentration of the standard which were
diluted in a solvent, and the molar ratio of the analyte (M) to the internal standard (MO) was
plotted against the peak area ratio of the two components (S/SO) (equation 2.16). The gradient
of the straight line obtained is the calibration factor K.
M/Mo = K × S/So (2.16)
The calibrations were carried out using decane and dodecane as GC standards and toluene,
methanol and acetone as solvents.
69
Table 2.1 Molecular weights, boiling points, retention times and calibration factors for all
components involved in the gas phase hydrodeoxygenation of MIBK, acetone and DIBK.
a) Acetone was used as a solvent.
b) Calibration factors were estimated using decane as a standard related to the effective
carbon-atom number.
c) Toluene was used as a solvent.
Compound
M wt
(g mol-1)
Boiling
point (°C)
Retention
time (min)
K
(rel. to
decane)
K
(rel.to
corresponding
ketones (MIBK,
acetone and
DIBK)
MIBKa 100
117 3.0 1.85 1.00
2-Methylpentanea 86
60 1.2 1.86 1.01
4-Methyl-2-
pentanola
102 131 4.4 1.77 0.96
2-Methyl-3-
pentanola
102 131 4.3 1.44 0.78
Acetoneb 58 56 1.7 5.00 1.00
Iso-Propanolb 60 83 2.3 4.75 0.95
Propaneb 44 -42 1.2 3.33 0.67
DIBKc 142 168 4.6 1.59 1.00
2,6-
Dimethylheptanec
128 135 1.7 1.33 0.84
2,6-Dimethyl-4-
heptanolc
144 179 6.0 1.30 0.82
70
Figure 2.11 GC trace for MIBK hydrodeoxygenation over 0.5%Pt/CsHPW at 40 °C.
4.5 min 220 °C
20 °C min-1
40 °C 1.5 min
Injector temperature = 250 °C
Detector temperature = 250 °C
Figure 2.12 Conditions of GC analysis using column A for all reactions studied.
2-Methyl pentane
MIBK
4-Methyl-2-pentanol
71
Figure 2.13 Calibration of MIBK. Figure 2.14 Calibration of 2-methyl pentane.
Figure 2.15 Calibration of 4-methyl-2-pentanol. Figure 2.16 Calibration of 2-methyl-3-pentanol.
y = 1.8508x R² = 0.9474
0
0.5
1
1.5
2
2.5
3
3.5
4
0 1 2 3
M/M
O
S/SO
y = 1.8602x R² = 0.9948
0
0.5
1
1.5
2
2.5
3
3.5
4
0 0.5 1 1.5 2
M/M
O
S/SO
y = 1.768xR² = 0.968
0
0.5
1
1.5
2
2.5
3
3.5
4
0 1 2 3
M/M
O
S/SO
y = 1.4381x R² = 0.9991
0
0.5
1
1.5
2
2.5
3
3.5
4
0 1 2 3
M/M
O
S/SO
72
Table 2.2 Molecular weights, boiling points, retention times and calibration factors for all
components involved in the gas phase hydrodeoxygenation of 3-pentanone, 2-hexanone,
cyclohexanone, 2-octanone, 2-butanone and acetophenone.
a) Acetone was used as a solvent.
b) Toluene was used as a solvent.
c) Methanol was used as a solvent.
d) Calibration factors were estimated using dodecane as a standard related to the
effective carbon-atom number.
Compound
M wt
(g mol-1)
Boiling
point (°C)
Retention
time (min)
K
(rel. to
dodecane)
K
(rel.to corresponding
ketones )
3-Pentanonea 86 101 2.7 1.51 1.00
Pentanea 72 36 1.2 1.36 0.90
3-Pentanola 88 115 3.9 1.22 0.81
2-Hexanonea 100 128 3.7 1.88 1.00
2-Hexanola 102 140 4.9 1.41 0.75
Hexaneb 86 69 1.2 1.52 0.81
Cycloheanonec 98 156 5.7 2.44 1.00
Cyclohexanolb 100 162 6.6 1.70 0.70
Cyclohexaneb 84 81 1.4 1.98 0.81
2-Octanonec 128 173 5.6 1.60 1.00
2-Octanolc 130 195 6.6 1.36 0.85
Octaneb 114 126 1.6 1.54 0.96
2-Butanoned 72 80 2.3 4.00 1.00
2-Butanold 74 99 3.2 3.70 0.93
Butaned 58 0 1.2 3.00 0.75
Acetophenonea 120 202 8.7 1.22 1.00
Ethylcyclohexanea 112 132 2.0 1.46 1.20
Ethylbenzenea 106 136 4.0 1.13 0.93
73
Figure 2.17 GC trace for acetophenone hydrodeoxygenation over 7%Pt/C+CsPW(1:19) at
100◦C.
Figure 2.18 Calibration of acetophenone. Figure 2.19 Calibration of ethylcyclohexane.
y = 1.2157xR² = 0.9883
0
1
2
3
4
5
6
7
0 2 4 6
M/M
0
S/S0
y = 1.462xR² = 0.9373
0
0.5
1
1.5
2
2.5
3
3.5
4
0 1 2 3
M/M
0
S/S0
Ethylcyclohexane
Ethylbenzene Acetophenone
74
Figure 2.20 Calibration of ethylbenzene.
Figure 2.21 GC trace for 3-pentanone hydrodeoxygenation over 0.5%Pt/CsHPW at 60◦C.
y = 1.1268xR² = 0.9784
0
1
2
3
4
5
0 1 2 3 4 5
M/M
0
S/S0
Pentane
3-Pentanone
3-Pentanol
75
Figure 2.22 Calibration of pentane. Figure 2.23 Calibration of 3-pentanol.
Figure 2.24 Calibration of 3-pentanone.
Table 2.3 Retention times and calibration factors for all components involved in conversion of
ethyl propanoate.
a) Acetone was used as a solvent.
b) Toluene was used as a solvent.
c) Retention times using column B.
d) Calibration factors were estimated using decane as standard related to the effective
carbon-atom number.
y = 1.3645xR² = 0.9598
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
0 1 2 3 4
M/M
0
S/S0
y = 1.218xR² = 0.9509
00.5
11.5
22.5
33.5
44.5
5
0 1 2 3 4
M/M
0
S/S0
y = 1.5074xR² = 0.9638
0
1
2
3
4
5
6
0 1 2 3 4
M/M
0
S/S0
Compound M wt
(g mol-1)
Boiling
point (◦C)
Retention
time
(min)
K
(rel. to
decane)
K
(rel. to
ethyl propanoate)
Ethyl propanoatea 102 99 2.5 2.21 1.00
Propanoic acidb 74 141 7.4 4.88 2.21
C2 [ethane+ethane] 30 -89 4.7-4.8c 5.00d 2.26
76
Figure 2.25 GC trace for ethyl propanoate hydrodeoxygenation over CsHPW at 180oC under
H2 using column B.
Figure 2.26 Condition of column B of GC analysis for all reactions studied.
0.5min 80°C
20°C min-1
180°C 2 min
20°C min-1
5 min 230°C
Ethane
Ethene
77
Figure 2.27 Calibration of propanoic acid. Figure 2.28 Calibration of ethyl propanoate.
Table 2.4 Retention times and calibration factors for all components involved in conversion of
anisole.
a) Acetone was used as a solvent.
b) Toluene was used as a solvent.
y = 4.8838xR² = 0.9917
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
0 0.5 1
M/M
0
S/S0
y = 2.206xR² = 0.9576
0
0.5
1
1.5
2
2.5
3
3.5
4
0 0.5 1 1.5 2
M/M
0
S/S0
Compound M wt
(g mol-1)
Boiling
point (◦C)
Retention
time (min)
K
(rel. to
dodecane)
K
(rel. to
anisole)
Anisolea 108 154 6.1 1.00 1.00
Cyclohexaneb 84 81 1.4 1.98 1.98
Methanolb 32 65 2.1 12.9 12.9
Cyclohexanolb 100 162 6.7 1.70 1.70
Benzenea 78 80 2.4 1.16 1.16
Toluenea 92 111 3.4 1.06 1.06
78
Figure 2.29 GC trace for anisole hydrodeoxygenation over 10%Cu +CsPW at 100◦C.
Figure 2.30 Calibration of anisole. Figure 2.31 Calibration of cyclohexane.
y = 1.0013xR² = 0.9816
0
1
2
3
4
5
6
0 1 2 3 4 5 6
M/M
0
S/S0
y = 1.9805xR² = 0.9732
0
0.5
1
1.5
2
2.5
3
3.5
0 1 2
M/M
0
S/S0
Anisole
Cyclohexanol
Benzene
Methanol
Cyclohexane
Toluene
79
Figure 2.32 Calibration of cyclohexanol. Figure 2.33 Calibration of toluene.
Figure 2.34 Calibration of benzene.
Table 2.5 Retention times and calibration factors for all components involved in conversion of
diisopropyl ether.
a) Calibration factors were estimated using decane as standard related to the effective
carbon-atom number.
b) Retention times using column B.
y = 1.6998xR² = 0.9961
0
1
2
3
4
5
0 1 2 3
M/M
0
S/S0
y = 1.0605xR² = 0.9733
0
1
2
3
4
5
6
0 2 4 6
M/M
0
S/S0
y = 1.163xR² = 0.9851
0
1
2
3
4
5
6
7
0 2 4 6
M/M
0
S/S0
Compound M wt
(g mol-1)
Boiling
point (°C)
Retention
time (min)
K
(rel. to
decane)a
K
(rel. to
diisopropyl
ether)
Diisopropyl
ether
102.18 69 1.3 2.00 1.00
Iso-Propanol 60.10 83 2.3 4.44 2.22
C3 44.10 -42 5.4-5.9b 3.33 1.67
80
Figure 2.35 GC trace for diisopropyl ether hydrodeoxygenation over 7%Pt/C+CsPW [1:19] at
110 ◦C under H2 using column A.
Figure 2.36 GC chromatogram for light hydrocarbon from hyrodeoxygenation of diisopropyl
ether over 7%Pt/C+CsPW [1:19] at 110◦C under H2 using column B.
Diisopropyl ether
Iso-propanal
C3
Propane
81
2.7 References
1. Y. Izumi, M. Ono, M. Kitagawa, M. Yoshida, K. Urabe, Microporous Mater. 5 (1995)
255.
2. M. A. Alotaibi, E. F. Kozhevnikova, I. V. Kozhevnikov, Appl. Catal. A 447-448
(2012) 32.
3. A. M. Alsalme, P. V. Wiper, Y. Z. Khimyak, E. F. Kozhevnikova, I. V. Kozhevnikov,
J. Catal. 276 (2010) 181.
4. G. C. Bond, S. J. Frodsham, P. Jubb, E. F. Kozhevnikova, I. V. Kozhevnikov, J. Catal.
293 (2012) 158.
5. W. Alharbi, E. Brown, E. F. Kozhevnikova, I. V. Kozhevnikov, J. Catal. 319 (2014)
174.
6. E. F. Kozhevnikova, I. V. Kozhevnikov, J. Catal. 224 (2004) 164.
7. N. Uekawa, T. Kudo, F. Mori, Y. J. Wu, K. Kakeawa, J. Colloid Interface Sci. 264
(2003) 378.
8. G. Leofanti, M. Padovan, G. Tozzola, B. Venturelli, Catal. Today. 41 (1998) 207.
9. J. M. Thomas, W. J. Thomas, Principles and Practice of Heterogeneous Catalysis,
VCH, 1997.
10. G. Attard, C. Barnes, Surfaces, Oxford University Press, 1998.
11. R. A. van Santen, P. W. N. M. van Leeuwen, J.A. Moulijn, B.A. Averill (Eds.)
Catalysis: An Integrated Approach, Elsevier, 2000.
12. S. Brunauer, P. H. Emmett, E. Teller, J. Am. Chem. Soc. 60 (1938) 309.
13. A. A. Mirzaei, PhD thesis, University of Liverpool, 1998.
14. D. Kealey, D. J. Haines, Analytical Chemistry, 1st ed., Bio scientific, Oxford, 2002.
15. P. W. Atkins, Physical Chemistry, Oxford University Press, 1998.
16. A. W. Burton, Zeolite Characterization and Catalysis: A Tutorial (2009) 1.
17. J. H. Sinfelt, Rev. Mod. Phys. 51 (1979) 569.
18. G. Bergeret, P. Gallezot, Handbook of Heterogeneous Catalysis, Wiley-VCH Verlag
GmbH & Co. KGaA, Weinheim, Germany, 2008.
19. J. E. Benson, M. Boudart, J. Catal. 4 (1965) 704.
20. J. E. Benson, H. S. Hwang, M. Boudart, J. Catal. 30 (1973) 146.
21. K. C. Taylor, J. Catal. 38 (1975) 299.
22. S. E. Wanke, N. A. Dougharty, J. Catal. 24 (1972) 367.
23. PD/TPR 2900 analyser, Operator's Manual, VI.02, April 1993.
82
24. P. Patnaik, Dean's Analytical Chemistry Handbook, 2nd ed., 2004.
25. E. A. Varella, Conservation Science for the Cultural Heritage: Applications of
Instrumental Analysis, Vol. 79, Springer-Verlag Berlin Heidelberg, 2013.
26. http://www.setaram.com/C80-Cells.htm.
27. B. Imelik, J. C. Vedrine, Catalyst Characterization, Physical Techniques for Solid
Materials, Plenum Press, New York, 1994.
28. F. W. Fifield, D. Kealy, Principles and Practice of Analytical Chemistry, (5th ed)
Blackwell Science Ltd, 2000.
29. L. M. Harwood, C. J. Moody, J. M. Percy, Experimental Organic Chemistry: Standard
and Microscale, Blackwell Science, 2001.
30. F. Al-Wadaani, E. F. Kozhevnikova, I. V. Kozhevnikov, J.Catal. 257 (2008) 199.
31. T. Pham, D. Shi, D. Resasco, Topics in Catalysis. 57 (2014) 706.
32. V. V. Costa, H. Bayahia, E. F. Kozhevnikova, E. V. Gusevskaya, I. V. Kozhevnikov,
ChemCatChem. 6 (2014) 2134.
33. E. F. Kozhevnikova, PhD thesis, University of Liverpool, 2004.
34. J. Mendham, R. C. Denney, J. D. Barnes, M. J. K. Thomas, Vogel’s Textbook of
Quantitative Chemical Analysis, Pearson Education Ltd., 2000.
83
3. Catalyst characterisation
3.1 Introduction
This chapter will present and discuss the results of catalysts characterisation. The catalyst’s
surface and porosity properties are investigated by BET. The metals supported on the surface of
Cs2.5PW and active carbon are probed using XRD and gas chemisorption for the purpose of
determining the dispersion and average particle size. Catalyst stability, catalyst composition and
the strength and nature of acidic sites are also investigated using ICP, TGA, FTIR and ammonia
adsorption microcalorimetry.
3.2 Thermogravimetric analysis
TGA was utilized to determine the content of water in catalysts and to access their thermal
stability. Moreover, this technique was also used to characterise the composition of metal
precursors that were used to prepare metal-modified CsPW catalysts.
Previous studies showed that, the CsPW is insoluble in water and stable up to 600 °C, which
means that it has higher thermal stability than the parent HPW [1, 2]. The thermal analysis of
CsPW, 0.5%Pt/CsPW and 5%Ru/CsPW are presented in Figures 3.1-3.3. The experiment was
carried with a rate of heating of 20 °C per minute, under continuous N2 flow, and 0.015-0.025 g
of the sample weight pretreated at 150°C and 0.5 Torr pressure.
It can be seen in Figure 3.1 that approximately 3% loss in the weight occurred in the temperature
range up to 300 °C for CsPW catalyst corresponding to the loss of 3 to 4 molecules of water per
Keggin unit as physisorbed water and/ or crystallisation water [3]. More weight loss occurred at
around 580°C owing to the deprotonation of the catalyst and corresponds to 0.25 H2O molecules
per Keggin unit. This is shown in Equation 3.1.
84
Cs2.5 H0.5PW12O40 Cs2.5PW12O39.75 + 0.25 H2O (3.1)
TGA profiles of 0.5%Pt/CsPW and 5%Ru/CsPW-I are shown in Figure 3.2 and 3.3, respectively.
The weight loss of these catalysts is less than the parent CsPW. This is probably the result of the
reduction at 250°C of these catalysts before TGA analysis.
Figure 3.1 TGA of CsPW calcined under vacuum at 150°C for 1.5 h.
96
97
98
99
100
20 120 220 320 420 520 620 720
WE
IGH
T (
%)
TEMPERATURE (OC)
85
Figure 3.2 TGA analysis of 0.5%Pt/CsPW reduced at 250 °C in H2 for 2 h.
Figure 3.3 TGA analysis of 5.0%Ru/CsPW-I reduced at 250°C in H2 for 2 h.
96
97
98
99
100
20 120 220 320 420 520 620
WE
IGH
T (
%)
TEMPERATURE (°C)
96
97
98
99
100
20 120 220 320 420 520 620 720
WIG
HT
(%)
TEMPERATURE (OC)
86
3.3 Surface area and porosity studies
Nitrogen adsorption at -196 ºC represents the most widely used method to measure the catalyst
surface area and its porous texture. The nitrogen isotherm is obtained by plotting the volume of
nitrogen adsorbed against its relative pressure. Isotherm shape depends on the porous texture of
the catalyst. As exhibited in Figure 3.4, there are six types of isotherm that can be distinguished
according to IUPAC classification, but only types I, II, IV, and VI are generally found in catalyst
characterisation [4-6].
Type I, II and VI isotherms are representative of microporous, macroporous and uniform
ultramicroporous solids, respectively. Type IV isotherm is attributed to mesoporous materials,
and it is this isotherm that will be explained in more detail because of its relevance to the catalysts
prepared in this study.
Figure 3.4 The six types of adsorption isotherms [4].
87
On a mesoporous solid (type IV) a monolayer adsorption occurs at low pressure, while at high
relative pressures a multilayer adsorption takes place until condensation occurs, giving a sharp
adsorption volume increase. Adsorption continues on the external catalyst surface as mesopores
are completely saturated [4, 5]. HPA catalysts belong to this class of solid.
After saturation is reached, desorption of the adsorbed nitrogen takes place in the opposite
process to that of adsorption. For mesoporous solids, this process occurs at a pressure lower than
that for adsorption, leading to a hysteresis loop. There are four hysteresis types that are
recognised for solid catalyst, which are displayed in Figure 3.5 [5].
Figure 3.5 The four hysteresis shapes usually found by N2 adsorption [5].
Types H1 and H2 hysteresis are associated with solid materials that consist of particles
crossed by nearly cylindrical channels in shape, or of spheroidal aggregates or agglomerates.
Pores of uniform size and shape give H1 hysteresis, while H2 hysteresis occurs in non-uniform
pores. These hysteresises are due to a different size of pore mouth and pore body (this is in the
case of ink-bottle-shaped pores) and/or a differences in adsorption and desorption behavior close
to cylindrical through pores. Most common mesoporous materials such as HPAs usually show
type H1 or H2 hysteresis [5].
Types H3 and H4 hysteresis isotherms occur when the sample pores are uniform and non-uniform
in size and shape, respectively. These two hysteresis loops are usually formed when solid
88
materials consist of aggregates or agglomerates of particles creating slit-shaped pores (plates
or edged particles like cubes). Typical examples of this hysteresis category are shown by active
carbon and zeolites [5].
When solid materials have blind cylindrical, wedge-shaped and cone-shaped pores, no hysteresis
loop is observed. However, we can only observe materials with much reduced hysteresis loops
due to their irregular pores [5].
The general method for the measurement of surface area and porosity is described in Section
2.4.1. The Brunauer-Emmett-Teller (BET) method was used to calculate the total catalyst
surface area [7], while the Barrett, Joyner and Halenda (BJH) method was used to determine
pore size distribution and total pore volumes [8].
The surface area of water-soluble bulk HPW is very low, in the range of 1-10 m2/g [1, 9, 10].
However, salts of HPA with large cations, such as Cs+, are water insoluble and have a surface
area > 100 m2 g-1 [1, 11]. Table 3.1 and 3.2 show the BET surface area and porous texture of
the bifunctional metal−acid and acid catalysts, respectively, used in this work. Previous studies
showed that the introduction of metal into the structure reduces the surface area to a small extent,
which is the evidence of metal situated on the catalyst surface [12, 13]. To examine the effect
of catalyst preparation on catalyst activity the preparation procedure was varied regarding the
use of different metal precursors and impregnation conditions (see Section 2.3). The metal
loading of Pt and Ru was 0.5 and 5%, respectively, and 10% for Cu and Ni catalysts due to
lower catalytic activity of Ru, Cu and Ni compared to Pt. As can be seen from Table 3.1, the
catalyst preparation procedure had little effect on the catalyst texture, whereas the metal loading
had significant effect on the catalyst surface area. The catalysts had surface areas between 35
and 144 m2g-1 and low porosities typical of CsPW-based catalysts. 0.5% Pt and 5% Ru catalysts
had the surface areas above 100 m2g-1, whereas 10% Ni and Cu catalysts had lower surface
areas, 74-93 m2g-1 for Ni and 35 m2g-1 for Cu catalysts. As can be seen in Table 3.2, supporting
89
HPA onto high surface area supports increases the surface area of HPA catalysts, as compared
to bulk HPAs. These catalysts are mesoporous solids with an average pore diameter calculated
to be in the range of 22-185 Å.
Table 3.1 Texture of metal catalysts from N2 adsorption.
Catalyst SBETa
(m2g-1)
Pore volumeb
(cm3g-1)
Pore sizec
(Å)
0.5%Pt/CsPW 128 0.100 30
0.5%Pt/CsPW-I 108 0.091 34
0.5%Pt/CsPW-A 144 0.085 24
5.0%Ru/CsPW-I 103 0.088 34
5.0%Ru/CsPW-A 116 0.084 29
10%Cu/CsPW-I 35 0.044 51
10%Cu/CsPW-A 35 0.023 27
10%Ni/CsPW-I 93 0.090 39
10%Ni/CsPW-A 74 0.049 26
7%Pt/C 801 0.580 29
a) BET surface area.
b) Single point total pore volume.
c) Average BET pore diameter.
90
Table 3.2 Texture of acid catalysts from N2 adsorption [14].
a) HPA catalysts calcined under vacuum at 150 °C for 1.5 h. ZrO2 and Nb2O5 supports
were prepared in-house and calcined at 400 °C in air for 5 h.
b) BET surface area.
c) Single point total pore volume.
d) Average BET pore diameter.
Figure 3.6 and Figure 3.7 exhibit the BET isotherm and the pore size distribution of CsPW
respectively. It can be seen that, CsPW exhibited a type IV isotherm, in agreement with previous
studies [2, 5, 9, 15]. This type of isotherm is typical of mesoporous materials (2 nm < pore
diameter < 50 nm). In this case an H2 hysteresis loop was observed, indicating the presence of
mesopores of non-uniform shape. The mesopore-size distribution of CsPW gained form the BJH
method showed a sharp peak at around 40 Å diameter (Figure 3.7) in good agreement with that
reported previously by Okuhara [2]. The pore size distribution created using the N2 adsorption
technique employed in this study was not capable to account for micropores. However, the steep
Catalystsa SBETb
(m2g-1)
Pore volumec
(cm3g-1)
Pore sized
(Å)
H3PW12O40 2 0.04 81
H4SiW12O40 9 0.02 71
Cs2.25H0.75PW12O40 128 0.07 22
Cs2.5H0.5PW12O40 132 0.10 29
15%HPW/Nb2O5 126 0.11 34
15%HPW/ZrO2 109 0.09 36
15%HPW/TiO2 45 0.20 174
15%HPW/SiO2 202 1.00 169
15%HSiW/SiO2 221 1.02 185
91
increase in N2 adsorption at low pressure (P/P0 < 0.1) from the adsorption isotherm suggests the
existence of micropores in this catalyst as well [2].
Figures 3.8-3.16 represent the effect of metal loading on the adsorption isotherm of CsPW.
Different preparation procedures and metal precursors were used. However, no obvious change
in the N2 adsorption isotherms of Metal-CsPW catalysts was observed.
Figure 3.6 N2 adsorption-desorption isotherms on CsPW.
92
Figure 3.7 Pore size distribution of CsPW.
Figure 3.8 N2 adsorption-desorption isotherms on 0.5%Pt/CsPW.
93
Figure 3.9 N2 adsorption-desorption isotherms on 0.5%Pt/CsPW-I.
Figure 3.10 N2 adsorption-desorption isotherms on 0.5%Pt/CsPW-A.
94
Figure 3.11 N2 adsorption-desorption isotherms on 5%Ru/CsPW-I.
Figure 3.12 N2 adsorption-desorption isotherms on 5%Ru/CsPW-A.
95
Figure 3.13 N2 adsorption-desorption isotherms on 10%Ni/CsPW-I.
Figure 3.14 N2 adsorption-desorption isotherms on 10%Ni/CsPW-A.
96
Figure 3.15 N2 adsorption-desorption isotherms on 10%Cu/CsPW-I.
Figure 3.16 N2 adsorption-desorption isotherms on 10%Cu/CsPW-A.
97
3.4 Metal dispersion of bifunctional catalysts
0.5% Pt-, 5% Ru-, 10% Ni and 10% Cu-modified CsPW catalysts were used in the gas phase
hydrodeoxygenation of ketones, ethers and esters. The dispersions of the Pt- and Ru-modified
CsPW catalysts were determined using the pulse H2 chemisorption method. However, the
dispersions of 10% Cu/CsPW and 7% Pt/C were measured using XRD and CO chemisorption,
respectively. The general procedure for determination of metal dispersion using gas
chemisorption is described in detail in Sections 2.4.4 and 2.4.5.
Table 3.3 Particle size and dispersion of metals on CsPW.
Catalysta Db dc
(nm)
0.5%Pt/CsPW 0.46 2.0d
0.5%Pt/CsPW-I 0.10 9.0d
0.5%Pt/CsPW-A 0.11 8.2d
5.0%Ru/CsPW-I 0.048 19d
5.0%Ru/CsPW-A 0.054 17d
10%Cu/CsPW-I 0.019f 59e
10%Cu/CsPW-A 0.012f 88e
7%Pt/C 0.44g 2.0d
a) The metal loadings quoted were confirmed by the ICP-AES elemental analysis.
b) Metal dispersion in catalysts as determined from H2 chemisorption.
c) Metal particle diameter.
d) Values obtained from the equation d (nm) = 0.9/D [16].
e) From XRD.
f) Values obtained from the equation D = 1.1/d (nm) [17].
g) Form CO chemisorption.
Table 3.3 shows the metal dispersion, D, and metal particle diameter, d, in the catalysts. For
Pt/CsPW and Ru/CsW catalysts, these were determined from H2 titration and for Cu/CsPW from
98
XRD. The results for 0.5%Pt/CsPW catalysts demonstrate a strong effect of catalyst preparation
on the metal dispersion. The use of Pt(acac)2 as a platinum source and benzene as a solvent for
impregnation gave a Pt dispersion D = 0.46 corresponding to a Pt particle diameter d = 2.0 nm.
This dispersion was much higher than that obtained by impregnation with H2PtCl6 from aqueous
solution, i.e. D = 0.10 – 0.11 (d = 8.2 – 9.0 nm) for the Pt/CsPW-I and Pt/CsPW-A catalysts. The
mode of impregnation from aqueous media, i.e. with or without ageing of the PtII – CsPW
aqueous slurry, practically did not affect the Pt dispersion and particle size. The same was
observed for the Ru/CsPW-I and Ru/CsPW-A. On the other hand, Cu/CsPW-A had larger Cu
particles (88 nm), hence a lower Cu dispersion, than Cu/CsPW-I (59 nm) (Table 3.4).
For 7%Pt/C catalyst, Pt dispersion and particle size were determined from CO chemisorption (D
= 0.44±0.07, d = 2.0 nm, average of three measurements); these values are in agreement with
many literature reports. It should be noted that H2 titration overestimated Pt dispersion for this
catalyst to give a value of D = 1.2±0.1. This may be explained by hydrogen spillover onto the
carbon support, which has been reported previously ([18] and references therein). XRD was also
unable to determine correctly the size of Pt particles. 7%Pt/C exhibited a clear pattern of the Pt
fcc phase, with 39.8 (111), 46.3 (200) and 67.5 (220) reflections (Figure 3.21, Section 3.6), from
which the particle size was estimated to be d = 28±5 nm (average of three measurements, D =
0.032). This implies that the XRD was biased towards larger Pt particles.
Pt/CsPW, Au/CsPW and bimetallic Pt/Au/CsPW catalysts with metal loading between 0.3-6 wt%
were used to study the enhancing effect of gold in the hydrodeoxygenation of 3- pentanone. An
accurate assessment of metal dispersion was obtained from hydrogen adsorption. Moreover,
STEM and XRD were used to estimate metal particle size for some of these catalysts.
Table 3.4 shows the results of H2/O2 titration of CsPW-supported Pt and PtAu catalysts, which
was carried out at room temperature by the pulse method in flow system. Under such conditions,
99
CsPW did not adsorb any hydrogen, in agreement with the previous report [19]. No hydrogen
adsorption was observed on the Au/CsPW either, which is also in agreement with the literature
[20-22]. Hence the adsorption of H2 observed on the PtAu catalysts was attributed entirely to
platinum. Pt dispersion, D, in 0.32%Pt/CsPW was found to be 0.61 and predictably reduced to
0.19 in 5.8%Pt/CsPW, which corresponds to a Pt particle size of 1.5 and 4.7 nm in these catalysts,
respectively. The PtAu/CsPW catalysts prepared by co-impregnation showed a small reduction
trend in Pt dispersion in comparison with the unmodified Pt/CsPW catalysts, although the
difference was within the experimental error. This may be explained by PtAu alloying. Notably,
the Pt dispersion in the 0.32%Pt/0.36%Au/CsPW-SI catalyst prepared by sequential Au-after-Pt
impregnation dropped significantly down to 0.30 (Table 3.4). This may be the reason for the less
efficient performance of this catalyst as compared to the co-impregnation catalyst
0.28%Pt/0.35%Au/CsPW-CI (see Chapter 5).
100
Table 3.4 H2/O2 titration of catalysts.
Catalysta H2/Pttotal
(mol/mol)
Db
dc
(nm)
0.32%Pt/CsPW-I 0.91±0.13 0.61±0.09 1.5
0.28%Pt/0.35%Au/CsPW-CId
0.83±0.09 0.55±0.06
1.6
0.32%Pt/0.36%Au/CsPW-SIe
0.45±0.09 0.30±0.06
3.0
5.8%Pt/CsPW-I 0.28±0.07 0.19±0.05 4.7
5.6%Pt/4.3%Au/CsPW-CId
0.26±0.03 0.17±0.02 5.3
2.6%Au/CsPW 0f ≤10
CsPW 0f
a) Metal loadings obtained from ICP-AES analysis.
b) Pt dispersion determined as an average from three H2/O2 titration measurements
assuming negligible H2 adsorption on gold.
c) Metal particle diameter: for Pt from the equation d (nm) = 0.9/D, for Au from STEM.
d) Catalysts prepared by co-impregnation of H2PtCl6 and HAuCl4 followed by reduction
with H2 at 250 oC/2 h.
e) Catalyst prepared by sequential impregnation of H2PtCl6 then HAuCl4, with Pt(IV)
reduced to Pt(0) with H2 at 250 oC/2 h prior to HAuCl4 impregnation, then the
PtoAuIII/CsPW was reduced with H2 at 250 oC/2 h.
f) No H2 adsorption observed.
3.6 X-ray diffraction
Powder XRD patterns were recorded for metal-modified CsPW catalysts. The Scherrer equation
was used to determine metal particle size of these catalysts. Several researchers have reported
the XRD pattern for CsPW which was found to be similar to the parent HPW [2, 23]. Figure 3.17
shows the results of powder XRD analysis of CsPW-supported metal catalysts. The XRD of
0.5%Pt/CsPW displays only the well-known pattern of crystalline CsPW [2]; it did not reveal
any Pt metal phase, which is probably due to the fine dispersion and low concentration of Pt in
101
the catalyst. 10%Cu/CsPW-I and 10%Cu/CsPW-A exhibited sharp XRD pattern of Cu metal,
with (111) and (200) reflections at 43.1 and 50.3o, respectively, in agreement with the standard
values 43.3 and 50.4o (JCPDS, copper 04-0836). From these, a mean diameter of 59 - 88 nm for
Cu particles was obtained using the Scherrer equation. Previously, the Kozhevnikov group
measured the Cu dispersion using different techniques, such as XRD, TEM and N2O adsorption
[19]. TEM and XRD gave very close results. However, N2O titration gave agreement with TEM
and XRD only by careful optimisation of N2O concentration, titration temperature and the length
of exposure. The problem is that oxidation of Cu with N2O is not restricted to the outermost Cu
layer. Consequently, XRD is as a more reliable and less time consuming technique for measuring
the Cu dispersion. The XRD of 10%Ni/CsPW-I and 10%Ni/CsPW-A did not reveal any Ni metal
phase (Figure 3.17), which prevented the determination of Ni dispersion in these catalysts. The
absence of Ni phase may be explained by Ni oxidation to NiO on exposure of these catalysts to
air [24]. It should be noted, however, that no XRD pattern of NiO was observed either (Figure
3.17), which may be explained by fine dispersion of the NiO in our catalysts.
Figure 3.17 XRD patterns (CuKα radiation) for fresh catalysts: (a) 0.5%Pt/CsPW, (b)
10%Cu/CsPW-I, (c) 10%Cu/CsPW-A, (d) 10%Ni/CsPW-I, (e) 10%Ni/CsPW-A, (f) 7%Pt/C.
25 30 35 40 45 50 55 60 65 70
2 Theta (deg)
ab
c
d
e
f
102
3.7 Fourier transform infrared spectroscopy (FTIR)
FTIR technique was utilised in this study to investigate the Keggin structure of fresh and spent
HPA catalysts. Also it was used to study the Lewis and Bronsted acid sites on the catalyst surface.
The general method of FTIR measurements is described in Section 2.4.10.
3.7.1 Keggin structure
IR active bands of the Keggin anion are found in the region between 1200 and 500 cm-1 [10, 25].
Choi et al. [26] reported infrared bands of bulk HPW and CsPW after calcination at 3000C for 2
h at 984 cm-1 (terminal W=O), 1080 cm-1 (P-O in the central tetrahedron), 897, and 812 cm-1 (W-
O-W) related to asymmetric vibrations in the Keggin polyanion. Figure 3.18 exhibits the DRIFT
spectrum of the CsPW catalyst.
In this study, we examined the fresh and spent 0.5%Pt/CsPW catalysts to investigate any possible
change in the catalyst structure after MIBK hydrodeoxygenation at 1000C under H2 in the gas
phase (Figure 3.19). These two spectra show absorption bands of Keggin structure before and
after use which indicate the stability of this catalyst structure under the reaction conditions.
Moreover, the infrared spectra of CsPW and 7%Pt/C+CsPW (1:19) spent catalysts after
diisopropyl ether hydrogenation show no sign of structural changes (Figure 3.20 and 3.21).
We also investigated the structure stability of spent 7% Pt/C+CsPW(1:19) after gas phase
ethyl propanoate reaction under H2 at 200 0C (Figure 3.22). This catalyst also exhibited the well-
known IR spectrum of the Keggin anion at 1079, 987, 889 and 809 cm-1, matching the spectrum
of the fresh CsPW catalyst (Figure 3.18).
103
Figure 3.18 DRIFT spectra for fresh CsPW catalyst.
Figure 3.19 DRIFT spectra for (a) fresh and (b) spent 0.5%Pt/CsPW catalyst after MIBK
hydroxygenation at 100 0C under H2, catalyst reduced under H2 at 250 0C for 1.5 h.
b
a
Wavenumber (cm-1)
Ref
lecta
nce
→
Wavenumber (cm-1)
Ref
lecta
nce
→
104
Figure 3.20 DRIFT spectra for spent CsPW catalyst after diisopropyl ether hydroxygenation at
150 0C under N2.
Figure 3.21 DRIFT spectra for spent 7%Pt/C+CsPW(1:19) catalyst after diisopropyl ether
hydroxygenation at 110 0C under H2.
Wavenumber (cm-1)
Ref
lecta
nce
→
Wavenumber (cm-1)
Ref
lecta
nce
→
105
Figure 3.22 DRIFT spectra for spent 7%Pt/C+CsPW(1:19) catalyst after ethyl propanoate
hydroxygenation at 200 0C under H2.
3.7.2 Pyridine adsorption
Adsorption of pyridine as a base on surface acid sites is fundamental technique used for
characterisation of the nature of surface acid sites in heterogeneous catalysis [27-31]. Pyridine
chemisorbs on Brønsted and Lewis acid sites and displays a vibration at around 1540, 1490 and
1450 cm-1 which are attributed to Brønsted, Brønsted and Lewis and Lewis acidic sites,
respectively [32].
The FTIR of pyridine adsorption was utilised in this study to characterise the nature of the acidity
of CsPW and 0.5%Pt/CsPW catalysts. Previous studies showed that bulk HPW and CsPW, pre-
treated below 300°C, possess a very strong Brønsted acidity [1, 10]. From the DRIFT spectra
(Figure 3.23), both CsPW and 0.5%Pt/CsPW catalysts have both Lewis and Brønsted acid sites
as confirmed by IR bands at 1450 and 1540 cm-1 respectively. This is in agreement with previous
studies [28]. Modification of CsPW with Pt may reduce the strength Brønsted sites, which could
be expected due to the interaction between the metal and acid sites [33, 34].
Wavenumber (cm-1)
Ref
lecta
nce
→
106
Figure 3.23 DRIFT spectra of adsorbed pyridine on (a) CsPW and (b) 0.5%Pt/CsPW catalysts.
3.8 Microcalorimetry of ammonia adsorption
The acid strength of supported and bulk acid catalysts was measured using ammonia adsorption
microcalorimetry technique. Specifically, we looked at the correlation between the turnover
reaction rate (turnover frequency) of ethyl propanoate (EP) and diisopropyl ether (DPE)
decomposition and the HPA acid strength, which can be used to predict the activity of acid
catalysts in these reactions. Solid acid catalysts under study are based on Keggin-type tungsten
HPAs, H3PW12O40 and H4SiW12O40, and possess predominantly Brønsted acid sites. Previously,
the effect of HPA catalyst acid strength on turnover rate of alcohols dehydration has been studied
[14, 35, 36]. In cited references, these catalysts have been thoroughly characterised using XRD,
FTIR, FTIR of adsorbed pyridine, 31P MAS NMR and NH3 adsorption calorimetry, and their
properties have been discussed in detail [14, 35-37]. Experimental procedure of ammonia
adsorption is detailed in Section 2.4.8. The acid strength of the catalysts under study decreases
in the order (Table 3.5): H3PW12O40 > H4SiW12O40 > Cs2.5H0.5PW12O40 > Cs2.25H0.75PW12O40 >
b
a
Ab
sorb
an
ce→
Wavenumber (cm-1)
107
15%H3PW12O40/SiO2 ≈ 15%H4SiW12O40/SiO2> 15%H3PW12O40/TiO2 > 15%H3PW12O40/Nb2O5
> 15%H3PW12O40/ZrO2. This order is in line with the catalytic activity (see Chapter 6).
Table 3.5 Initial enthalpy of ammonia adsorption at 150 oC.
Catalystsa ΔHNH3
(kJ mol-1)
H3PW12O40 -197
H4SiW12O40 -171
Cs2.25H0.75PW12O40 -162
Cs2.5PW12O40 -164
15%HPW/Nb2O5 -132
15%HPW/ZrO2 -121
15%HPW/TiO2 -143
15%HPW/SiO2 -154
15%HSiW/SiO2 -154
a All HPA catalysts calcined at 150 oC under vacuum for 1.5 h; in-house made supports ZrO2
and Nb2O5 calcined at 400 oC in air for 5 h [14].
3.9 Conclusion
This chapter has provided the results of catalyst characterization. The nitrogen physisorption
technique was used for measuring surface area and porosity of catalysts. All the bifunctional
metal−acid and acid catalysts were mesoporous solids with average pore diameters of 22-185 Å.
For bifunctional metal acid catalysts, catalyst preparation procedure had little effect on the
108
catalyst texture, whereas the metal loading had significant effect on the catalyst surface area.
Supporting HPA onto high surface area supports increases the catalyst surface area.
Gas chemisorption was used to determine the dispersion and particle size of Pt and Ru on CsPW.
However, XRD was used the determine the particle size of Cu
FTIR showed that CsPW and its modified metal catalysts retained their primary Keggin structure
in all reaction conditions used in this study. FTIR of pyridine adsorption indicated Brønsted and
Lewis sites of CsPW and Pt/CsPW catalysts.
Ammonia adsorption microcalorimetry was used to determine the initial enthalpy of NH3
adsorption, ΔH, for HPA catalysts. Their ΔH values are in the range from -121 to -197 kJ/mol.
109
3.10 References
1. I. V. Kozhevnikov, Catalysts for Fine Chemical Synthesis, Catalysis by
Polyoxometalates, Wiley 2002.
2. T. Okuhara, H. Watanabe, T. Nishimura, K. Inumaru, M. Misono, Chem. Mater. 12
(2000) 2230.
3. N. Essayem, G. Coudurier, M. Fournier, J. C. Vedrine, Catal. Lett. 34 (1995) 223.
4. G. Rothenberg, Catalysis: Concepts and Green Applications, Wiley-VCH, Weinheim,
2008.
5. G. Leofanti, M. Padovan, G. Tozzola, B. Venturelli, Catal. Today 41 (1998) 207.
6. S. J. Gregg, K. S. W. Sing, Adsorption, Surface Area and Porosity, Academic Press,
London, 1982.
7. S. Brunauer, P. H. Emmett, E. Teller, J. Am. Chem. Soc. 60 (1938) 309.
8. L. G. Joyner, E. P. Barrett, R. Skold, J. Am. Chem. Soc. 73 (1951) 3155.
9. N. Mizuno, M. Misono, Chem. Rev. 98 (1998) 199.
10. T. Okuhara, N. Mizuno, M. Misono, Adv. Catal. 41 (1996) 113.
11. I. V. Kozhevnikov, Chem. Rev. 98 (1998) 171.
12. K. Na, T. Okuhara, M. Misono, J. Catal. 170 (1997) 96.
13. E. F. Kozhevnikova, E. Rafiee, I. V. Kozhevnikov, Appl. Catal. A. 260 (2004) 25.
14. W. Alharbi, E. I. Kozhevnikova, I. V. Kozhevnikov, ACS Catal. 5 (2015) 7186.
15. T. Nakato, Y. Toyoshi, M. Kimura, T. Okuhara, Catal. Today 52 (1999) 23.
16. J. E. Benson, H. S. Hwang, M. Boudart, J. Catal. 30 (1973) 146.
17. A. Dandekar, M. A. Vannice, J. Catal. 178 (1998) 621.
18. G. Bergeret, P. Gallezot, in: G. Ertl, H. Knözinger, F. Schüth, J. Weitkamp (Eds.),
Handbook of Heterogeneous Catalysis, 2, Wiley-VCH, New York, 2008.
19. M. A. Alotaibi, E. F. Kozhevnikova, I. V. Kozhevnikov, Appl. Catal. A 447-448 (2012)
32.
20. C. Hoang-Van, G. Tournier, S. J. Teichner, J. Catal. 86 (1984) 210.
21. J. E. Benson, M. Boudart, J. Catal. 4 (1965) 704.
22. E. Bus, J. A. van Bokhoven, Phys. Chem. Chem. Phys. 9 (2007) 2894.
23. J. A. Dias, E. Caliman, S. C. L. Dias, Micropor. Mesopor. Mat. 76 (2004) 221.
24. M. L. Toebes, J. H. Bitter, A. J. van Dillen, K. P. de Jong, Catal. Today 76 (2002) 33.
25. J. B. Moffat, Metal-Oxygen Clusters. The Surface and Catalytic Properties of
Heteropoly Oxometalates, Kluwer, New York, 2001.
110
26. S. M. Choi, Y. Wang, Z.M. Nie, J. Liu, C. H. F. Peden, Catal. Today 55 (2000) 117.
27. B. M. Devassy, S. B. Halligudi, J. Catal. 236 (2005) 313.
28. B. M. Devassy, F. Lefebvre, S. B. Halligudi, J. Catal. 231 (2005) 1.
29. H. Sachsenroder, E. Brunner, M. Koch, H. Pfeifer, B. Staudte, Microporous Mater. 6
(1996) 341.
30. A. A. Khasin, T. M. Yur'eva, L. M. Plyasova, G. N. Kustova, H. Jobic, A. Ivanov, Y. A.
Chesalov, V. I. Zaikovskii, A. V. Khasin, L. P. Davydova, V. N. Parmon, Russ. J. Gen.
Chem. 78 (2008) 2203.
31. K. Okumura, K. Yamashita, M. Hirano, M. Niwa, J. Catal. 234 (2005) 300.
32. B. H. Davis, R. A. Keogh, S. Alerasool, D. J. Zalewski, D. E. Day, P. K. Doolin, J.
Catal. 183 (1999) 45.
33. D. Kubicka, N. Kumar, T. Venalainen, H. Karhu, I. Kubickova, H. Osterholm, D. Yu.
Murzin, J. Phys. Chem. B 110 (2006) 4937.
34. V. Nieminen, M. Kangas, T. Salmi, D. Yu. Murzin, Ind. Eng. Chem. Res. 44 (2005)
471.
35. A. M. Alsalme, P. V. Wiper, Y. Z. Khimyak, E. F. Kozhevnikova, I. V. Kozhevnikov, J.
Catal. 276 (2010) 181.
36. W. Alharbi, E. Brown, E. F. Kozhevnikova, I. V. Kozhevnikov, J. Catal. 319 (2014)
174.
37. G. C. Bond, S. J. Frodsham, P. Jubb, E. F. Kozhevnikova, I. V. Kozhevnikov, J. Catal.
293 (2012) 158.
111
4. Hydrogenation of ketones over
bifunctional Pt-heteropoly acid catalyst
in the gas phase
4.1 Introduction
Biomass-derived organic oxygenates such as ketones, carboxylic acids, alcohols, phenols, etc.,
readily available from natural resources, are attractive as renewable raw materials for the
production of value-added chemicals and bio-fuels, as a result of the decline in oil resources and
global warming [1, 2]. Today the main targets of biofuels are diesel, gasoline and jet fuel.
Commercially, bioethanol and fatty acid alkyl esters are used as biogasoline and biodiesel.
However, the drawbacks of this first generation of biofuels are their low stability, low energy
density and competition with food production. These drawbacks can be overcome by developing
next generation biofuels that do not conflict with food production and totally match with
petroleum based transportation fuels composed of hydrocarbons [3, 4]. Generally, biomass
derived organic compounds contain a large amount of oxygen and have low energy density. For
fuel applications, they require reduction in oxygen content to increase their caloric value. Much
attention is given to deoxygenation of organic oxygenates using heterogeneous catalysis, in
particular for the upgrading of biomass-derived oxygenates obtained from fermentation,
hydrolysis and fast pyrolysis of biomass [5-9].
Biomass-derived ketones can be further upgraded by aldol condensation and hydrogenation to
produce alkanes that fall in the gasoline/diesel range. The hydrogenation of ketones to produce
alcohols is feasible and is catalysed by supported metal catalysts (e.g. Pt/C and Pd/C) [10];
however, further hydrogenation to alkanes is rather difficult to achieve on such catalysts [11, 12].
In combination with acid catalysts (bifunctional metal-acid catalysts), the production of alkane
112
from ketone can be achieved much easier ([11-13] and references therein). This process occurs
via a sequence of steps involving hydrogenation of ketone to alcohol on metal sites followed by
dehydration of secondary alcohol to alkene on acid sites and finally hydrogenation of alkene to
alkane on metal sites (Scheme 4.1). Although alkenes have higher octane number than alkanes,
alkanes are more favourable for fuels because of their higher stability. Consequently, using
metal-acid bifunctional catalysts is one of the most promising approaches for complete
hydrodeoxygenation (HDO) [3].
Scheme 4.1 Ketone hydrogenation via bifunctional metal-acid catalysis.
Previously, Kozhevnikov group have reported that platinum on acidic supports, namely Pt on
zeolite HZSM-5 [11] and acidic caesium salt of tungstophosphoric heteropoly acid
Cs2.5H0.5PW12O40 (CsPW) [12], are active bifunctional catalysts for hydrogenation of methyl
isobutyl ketone (MIBK) to the corresponding alkane 2-methylpentane (MP) in the gas phase.
0.5%Pt/CsPW, possessing very strong Brønsted acidity in addition to Pt metal sites, has been
found to be particularly efficient catalyst giving 100% yield of MP at 100 oC and 1 bar pressure
without alkane isomerisation [12]. They have also proved the bifunctional metal-acid catalysis
mechanism (Scheme 4.1) for MIBK hydrodeoxygenation [12]. Recently, Mizuno et al. [13] have
applied this catalyst for hydrogenation of ketones, phenols and ethers in the liquid phase at 120
oC and 5 bar H2 pressure.
In this chapter, we investigate the hydrogenation (hydrodeoxygenation) of a variety of ketones
including aliphatic ketones and acetophenone in the gas phase using bifunctional metal-acid
catalysts comprising Pt, Ru, Ni and Cu supported on CsPW. Firstly, methyl isobutyl ketone
(MIBK) hydrogenation was studied in more detail using metal (Pt, Ru, Ni, Cu) modified CsPW
113
catalysts. Then the most efficient catalyst, 0.5%Pt/CsPW, was used to study the hydrogenation
of aliphatic ketones and acetophenone. It is demonstrated that 0.5%Pt/CsPW is a highly efficient
and versatile catalyst for the ketone-to-alkane hydrogenation, and an insight into the reaction
mechanism is gained.
4.2 Hydrogenation of MIBK over CsPW-supported metal catalysts
The hydrogenation of MIBK was carried out in the gas phase in flowing H2. The catalysts were
tested at 60-100 oC under atmospheric pressure in a Pyrex fixed-bed down-flow reactor (9 mm
internal diameter) fitted with an on-line gas chromatograph (Varian Star 3400 CX instrument
with a 30 m x 0.25 mm HP INNOWAX capillary column and a flame ionisation detector)
described in Chapter 2.
The catalysts studied together with their characterisation data are shown in Table 3.1 and 3.3 in
Chapter 3. To examine the effect of catalyst preparation on catalyst activity the preparation
procedure was varied regarding the use of different metal precursors and impregnation conditions
(Chapter 2 section 2.3). The metal loading of Pt, Ru was 0.5% and 5%, respectively, and 10%
for Cu and Ni catalyst due to lower catalytic activity of Ru, Cu and Ni compared to Pt. Previous
H2-TPR, XRD, and FTIR studies have shown that CsPW in Pt/CsPW and Pd/CsPW catalysts is
resistant to reduction by H2 below 600◦C, and the primary (Keggin) structure of CsPW is retained
in CsPW-supported Pt, Pd, and Cu catalysts after H2 treatment at 400◦C [14].
The hydrogenation of methyl isobutyl ketone (MIBK) was studied in more detail using CsPW-
supported Pt, Ru, Ni and Cu catalysts in order to optimise catalyst preparation and to gain an
insight into the reaction mechanism. Representative results are shown in Table 4.1. CsPW alone
in the absence of metal exhibited very low activity at 80-100 oC. As found previously [12], non-
acidic metal catalysts, such as Pt/C and Ru/C, are active in hydrogenation of MIBK to alcohol,
114
2-methyl-4-pentanol (MP-ol), at 100 oC, but further hydrogenation to alkane, 2-methypentane
(MP), becomes feasible only at temperatures as high as 300 oC. In contrast, bifunctional metal-
acid catalyst 0.5%Pt/CsPW, prepared from Pt(acac)2 in benzene solution, showed excellent
activity in MIBK hydrogenation, giving 100% MP yield at 100 oC and 1 bar pressure (Table 4.1).
No MP isomerisation was observed at this temperature. It can be seen that both 0.5%Pt/CsPW-I
and 0.5%Pt/CsPW-A, prepared from H2PtCl6 in aqueous solution, were less active than the
0.5%Pt/CsPW catalyst (cf. MIBK conversions at 60 and 80 oC). This may be explained by the
higher Pt dispersion in 0.5%Pt/CsPW (Table 3.3 in Chapter 3) and the presence of chloride in
0.5%Pt/CsPW-I and 0.5%Pt/CsPW-A. Reaction selectivity was greatly affected by the
temperature. At 60 oC, MP-ol was the main product, whereas at 100 oC, MP was formed in almost
100% yield, in agreement with the previous report [12]. This suggests the change of the rate-
limiting step with increasing the temperature (see below).
5%Ru/CsPW-I and 5%Ru/CsPW-A exhibited close activities in MIBK hydrogenation (Table
4.1). These catalysts matched 0.5%Pt/CsPW in activity and selectivity at 100 oC, but at a ten
times higher metal loading, in agreement with the previous report [12], yet they were
considerably less active than 0.5%Pt/CsPW at lower temperatures 60-80 oC. Ni/CsPW and
Cu/CsPW catalysts were much less active, showing a moderate activity at 350 oC, with Ni being
more active than Cu. The mode of preparation of these catalysts, i.e. with or without ageing of
CsPW and metal precursor aqueous slurry, had little effect on their performance. It should be
noted that since the non-metal doped CsPW support also showed some catalytic activity (Table
4.1) the activity of Ni/CsPW and Cu/CsPW could to some extent be attributed to the CsPW rather
than to the Ni and Cu. Therefore, the activity of the catalysts studied in terms of MIBK
conversion per unit metal weight decreased in the order: Pt > Ru >> Ni > Cu. The 0.5%Pt/CsPW
catalyst, prepared from Pt(acac)2 as a platinum source in benzene solution, showed the best
performance in the MIBK-to-MP hydrogenation.
115
Table 4.1 Hydrogenation of MIBK over bifunctional metal-acid catalysts.a
a) Reaction conditions: 0.2 g catalyst, 3.6% MIBK in H2 flow, 1 bar pressure, 20 mL min-1
flow rate, 2 h time on stream, catalyst pre-treatment at 100 oC/1 h in H2 flow.
b) C1-C5 cracking products, mainly propene and butenes, together with small amount of C6+
condensation products.
Catalyst
Temperature
(oC)
Conversion
(%)
Selectivity (mol%)
MP MP-ol Otherb
CsPW 80 1 27 0 73
100 3 22 0 78
0.5%Pt/CsPW 60 76 13 80 7
80 97 42 50 8
100 100 100 0 0
0.5%Pt/CsPW-I
60 32 41 49 10
80 88 89 11 0
100 98 98 0 2
0.5%Pt/CsPW-A 60 59 16 84 0
80 94 29 63 8
100 100 100 0 0
5%Ru/CsPW-I 60 36 24 76 0
80 71 75 25 0
100 96 100 0 0
5% Ru/ CsPW-A
60 31 8 92 0
80 66 29 67 4
100 90 98 2 0
10%Ni/CsPW-I 350 24 93 0 7
10%Ni/CsPW-A 350 19 95 0 5
10%Cu/CsPW-I 350 9 93 2 5
10%Cu/CsPW-A 350 4 97 0 3
116
Table 4.2 compares the performances of bifunctional catalyst 0.5%Pt/CsPW having metal and
acid sites in a rather close proximity to each other and the corresponding 1:19 w/w physical
mixture of 7%Pt/C and CsPW containing 0.35% of Pt, in which metal and acid sites are a longer
distance apart. It can be seen that these two catalysts give very similar MIBK conversions in the
temperature range of 60 – 100 oC. This indicates that the reaction is not limited by migration of
intermediates between metal and acid sites in the bifunctional catalyst [15], hence the metal and
acid sites are not required to be located in close proximity for the reaction to occur. It should be
noted, however, that the 0.5%Pt/CsPW catalyst produced more by-products (C1-C5 hydrocarbons
and C6+ condensation products) than the mixed 7%Pt/C + CsPW catalyst (Table 4.2).
Table 4.2 Hydrogenation of MIBK over Pt/CsPW and Pt/C+CsPW catalysts.a
Catalyst
Temperature
(oC)
Conversion
(%)
Selectivity (mol%)
MP MP-ol Otherb
0.5%Pt/CsPW
60 94 12 81 7
80 99 60 29 11
100 100 100 0 0
7%Pt/C+CsPWc
60 91 17 81 2
80 97 90 10 1
100 99 100 0 0
a) Reaction conditions: 0.2 g catalyst, 3.6% MIBK in H2 flow, 1 bar pressure, 20 mL min-1
flow rate, 2 h time on stream, catalyst pre-treatment at reaction temperature for1 h in H2
flow.
b) C1-C5 cracking products, mainly propene and butenes, together with small amount of C6+
condensation products.
c) Physical mixture of 7%Pt/C + CsPW (0.35% Pt content).
The time course shown in Figure 4.1 demonstrates stable catalytic activity of 0.5%Pt/CsPW in
MIBK hydrogenation for 2.5 h on stream. Previously, extended stability tests showed no catalyst
deactivation at least for 14 h on stream [12].
117
Figure 4.1 MIBK hydrogenation over 0.5%Pt/CsPW (0.2 g catalyst, 60 oC, 3.6% MIBK in H2
flow, 1 bar pressure, 20 mL min-1 flow rate, catalyst pre-treatment at 100 oC/1 h in H2 flow).
The activation energy of MIBK hydrogenation over 0.5%Pt/CsPW was determined under
differential conditions. To fit these conditions, the catalyst sample was reduced to 0.025 g diluted
with 0.175 g SiO2, and the flow rate was increased to 100 mL min-1. The reaction obeys the
Arrhenius equation with an activation energy Ea = 69 kJ mol-1 in the temperature range 80 – 110
oC where MP is by far the main reaction product (Figure 4.2). The high activation energy
indicates that the reaction occurred under kinetic control. This is supported by the Weisz-Prater
analysis [16] of the reaction system. Assuming spherical catalyst particles and Knudsen diffusion
regime, the Weisz-Prater criterion was calculated to be CWP = 1.2∙10-2 < 1 indicating no internal
diffusion limitations [17]. Other results reported previously [12], such as the close to zero
reaction order in MIBK and DIBK conversion scaling almost linearly with the Pt loading in the
catalyst, are also in agreement with reaction occurring under kinetic control.
0
20
40
60
80
100
0 50 100 150
Co
nv
ers
uin
& S
ele
cti
vit
y (
mo
l %
)
Time (min)
Conversion
2MP
MP-ol
other
118
Figure 4.2 Arrhenius plot for MIBK hydrogenation over 0.5%Pt/CsPW (0.025 g catalyst diluted
with 0.175 g SiO2, 3.6% MIBK in H2 flow, 1 bar pressure, 100 mL min-1 flow rate, MIBK
conversion range X = 3 – 17%).
4.3 Dehydration of 2-methyl-4-pentanol over CsPW
The dehydration of the secondary alcohol MP-ol is the second step in MIBK hydrogenation
through the bifunctional metal-acid-catalysed pathway (Scheme 4.1). It was studied to obtain
knowledge about the rate-limiting step in the MIBK hydrogenation.
Scheme 4.2 Dehydration of 2-methyl-4-pentanol over CsPW.
MP-ol dehydration over CsPW was found to yield two 2-methylpentene isomers with high
selectivity (Scheme 4.2). MP-ol conversion increased with increasing the temperature to reach
100% above 80 oC (Table 4.3). The reaction was found to be close to zero order in MP-ol (0.15
order) in the MP-ol partial pressure range of 1 – 3 kPa (Figure 4.3). Previously, zero order in
0
0.5
1
1.5
2
2.5
3
0.00255 0.0026 0.00265 0.0027 0.00275 0.0028 0.00285
Ln
X
1/T (K-1)
119
alcohol has been observed for isopropanol dehydration over CsPW [18]. The activation energy
of MP-ol dehydration was found to be Ea = 130 kJ mol-1 in the temperature range of 60 – 80 oC
(determined under differential conditions at MP-ol conversion X = 0.8 – 12%).
Table 4.3 Dehydration of MP-ol over CsPW.a
a) Reaction conditions: 0.2 g catalyst, 1.6% MP-ol in N2 flow, 1 bar pressure, 20 mL min-1
flow rate , 3 h time on stream, catalyst pre-treatment at reaction temperature for 1 h in
N2 flow.
Figure 4.3 Effect of MP-ol partial pressure on the rate of MP-ol dehydration over CsPW (0.025
g catalyst diluted with 0.175 g SiO2, 80 oC, 1 bar pressure, 100 ml min-1 N2 flow rate, X = 8 –
20%).
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0 0.5 1 1.5 2 2.5 3
Rat
e (
mo
l h-1
g-1)
P (kPa)
Temperature
(oC)
Conversion
(%)
Selectivity (mol%)
2-Methylpentene Other
40 3 79 21
60 43 97 3
80 99 100 0
100 100 100 0
120
Previously, it has been suggested that MIBK-to-MP hydrogenation over Pt/CsPW at 100 oC is
limited by the first step, i.e., hydrogenation of MIBK to MP-ol (Scheme 4.1). This is mainly
based on the fact that reaction rate scales with Pt loading, while MP selectivity remains constant
~100% [12]. The results on MP-ol dehydration obtained here fully support this view. First, the
MP-ol dehydration is fast at 100 oC; and second, it has much higher activation energy that the
MIBK-to-MP hydrogenation. Consequently, at lower temperatures, ~60 oC, MIBK-to-MP
hydrogenation appears to be limited by MP-ol dehydration, resulting in formation of MP-ol as
the main product. At higher temperatures, ~100 oC, the MP-ol dehydration step, having higher
activation energy, becomes faster than the MIBK-to-MP-ol hydrogenation step. The latter
becomes the rate-limiting step, which results in high MP selectivity. The last step in Scheme 1,
i.e., 2-methylpentene hydrogenation, appears to be fast, which is supported by the absence of 2-
methylpentenes in the products of MIBK hydrogenation. Alkene hydrogenation is known to be
significantly exothermic and fast, especially on Pt catalysts ([19] and references therein). The
heat of hydrogenation of 4-methyl-1-pentene has been reported to be -121 kJ mol-1 [19].
Therefore, hydrogenation of ketones to alkenes rather than to alkanes via metal-acid bifunctional
pathway on a single catalyst bed is not feasible. This could be better achieved by a consecutive
two-step process with ketone-to-alcohol hydrogenation on a metal catalyst as the first step
followed by alcohol dehydration on an acid catalyst as the second step.
121
4.4 Hydrogenation of aliphatic ketones over Pt/CsPW
Table 4.4 shows the results for hydrogenation of C3-C9 aliphatic ketones over 0.5%Pt/CsPW. It
can be seen that these ketones reacted very similarly to MIBK. At 60 oC, alcohols were the main
products, except for the higher C8-C9 ketones 2-octanone and diisobutyl ketone, which gave
mainly the corresponding alkanes. The latter indicates that the corresponding C8-C9 alcohols
were easier to dehydrate over CsPW as compared to the alcohols related to the lower ketones. At
100 oC, all ketones gave the corresponding alkanes in high yields 87 – 100%. Therefore,
0.5%Pt/CsPW is a versatile bifunctional catalyst for the gas-phase hydrogenation of aliphatic
ketones to alkanes.
122
Table 4.4 Hydrogenation of aliphatic ketones over 0.5%Pt/CsPW.a
Ketone
Temperature
(oC)
Conversion
(%)
Selectivity (mol%)
Alkane Alcohol Otherb
Acetone 60 62 5 70 25
100 98 87 9 4
2-Butanone 60 99 5 77 19
100 100 99 0 1
3-Pentanone 60 94 21 76 3
100 100 100 0 0
2-Hexanone 60 98 21 63 11
100 100 99 0 1
MIBK 60 96 18 77 5
100 100 100 0 0
Cyclohexanone 60 92 28 72 0
100 99 99 0 1
2-Octanonec 60 41 68 27 5
100 95 98 0 2
Diisobutyl
ketone
60 40 93 4 3
100 99 93 0 7
a) Reaction conditions: 0.2 g catalyst, 2.0% ketone in H2 flow, 1 bar pressure, 20 mL min-
1 flow rate, 3 h time on stream, catalyst pre-treatment at 100 oC/1 h in H2 flow.
b) C1-C5 cracking products together with small amount of ketone condensation products.
c) 5 h time on stream.
123
4.5 Hydrogenation of acetophenone over Pt/CsPW
Acetophenone is an example of aromatic ketone, and its hydrogenation is more complicated than
that of aliphatic ketones regarding the reaction selectivity and catalyst stability. The
hydrogenation of acetophenone through bifunctional metal-acid-catalysed pathway can be
represented by Scheme 4.3.
Scheme 4.3 Hydrogenation of acetophenone via bifunctional metal-acid catalysis.
Two bifunctional catalysts were tested at 100 oC: 0.5%Pt/CsPW and 1:19 w/w mixture of 7%Pt/C
and CsPW. The results are shown in Table 6. Both catalysts gave ethylcyclohexane and
ethylbenzene as the main products. It should be noted that no 1-phenylethanol and styrene was
observed amongst the products, which can be explained by their high reactivity. Thus, 1-
phenylethanol is known to be dehydrated much easier than aliphatic secondary alcohols [20].
Initially, 0.5%Pt/CsPW gave 98% selectivity to ethylcyclohexane at 74% acetophenone
conversion (73% yield). However, this catalyst suffered from deactivation, resulting in
significant loss in conversion and ethylcyclohexane selectivity over time on stream. After 6 h,
the conversion dropped to 59%, and ethylcyclohexane selectivity to 19% in favour of
ethylbenzene (81%). In contrast, the 7%Pt/C + CsPW mixed catalyst showed very little
deactivation with time, yielding 74-77% of ethylcyclohexane and only 3-5% of ethylbenzene
over 5 h on stream (Table 4.5). The poor stability of 0.5%Pt/CsPW to deactivation may be due
124
to blocking Pt sites by adsorption of acetophenone and/or ethylbenzene on the neighbouring
strong proton sites of CsPW.
Table 4.5 Hydrogenation of acetophenone.a
Catalyst
TOSb
(h)
Conversion
(%)
Selectivity (mol%)
Ethylcyclohexane Ethylbenzene
0.5%Pt/CsPW 3 74 98 2
0.5%Pt/CsPW 5 67 61 39
0.5%Pt/CsPW 6 59 19 81
7%Pt/C+CsPWc 3 80 96 4
7%Pt/C+CsPWc 5 79 94 6
a) Reaction conditions: 100 oC, 0.2 g catalyst, 0.5% acetophenone in H2 flow, 1 bar
pressure, 20 mL min-1 flow rate, 3 h time on stream, catalyst pre-treatment at 100 oC/1
h in H2 flow.
b) Time on stream.
c) Physical mixture of 0.02 g 7%Pt/C + CsPW (0.35% Pt content).
4.6 Conclusions
In this work, we have investigated the gas-phase hydrogenation of a wide range of ketones to
alkanes, including hydrogenation of aliphatic ketones and acetophenone, using bifunctional
metal-acid catalysis. The bifunctional catalysts comprise Pt, Ru, Ni and Cu metals supported on
acidic caesium salt of tungstophosphoric heteropoly acid Cs2.5H0.5PW12O40 (CsPW). The reaction
occurs via a sequence of steps involving hydrogenation of ketone to alcohol on metal sites
followed by dehydration of alcohol to alkene on acid sites and finally hydrogenation of alkene
to alkane on metal sites. Catalyst activity has been shown to decrease in the order: Pt > Ru >> Ni
> Cu. 0.5%Pt/CsPW has been demonstrated to be versatile catalyst for the hydrogenation of
aliphatic ketones, giving almost 100% alkane yield at 100 oC and 1 bar pressure. Evidence has
125
been provided that the reaction with Pt/CsPW at 100 oC is limited by ketone-to-alcohol
hydrogenation, whereas at lower temperatures (≤ 60 oC) by alcohol dehydration resulting in
alcohol formation as the main product. The catalyst comprising of a physical mixture of 7%Pt/C
+ CsPW has been found to be highly efficient as well, which indicates that the reaction is not
limited by migration of intermediates between metal and acid sites in the bifunctional catalyst.
The mixed 7%Pt/C + CsPW catalyst shows better performance stability in acetophenone
hydrogenation compared to the impregnated Pt/CsPW catalyst, which suffers from deactivation.
126
4.7 References
1. A. Corma, S. Iborra, A. Velty, Chem. Rev. 107 (2007) 2411.
2. E. L. Kunkes, D. A. Simonetti, R. M. West, J. C. Serrano-Ruiz, C. A. Gaertner, J. A.
Dumesic, Science 322 (2008) 417.
3. Y. Nakagawa, S. Liu, M. Tamura, K. Tomishige, ChemSusChem 8 (2015) 1114.
4. S. Lestari, P. Maki-Arvela, J. Beltramini, G. Q. Max Lu, D. Y. Murzin, ChemSusChem 2
(2009) 1109.
5. M. Snare, I. Kubickova, P. Maki-Arvela, K. Eranen, D. Yu. Murzin, Ind. Eng. Chem. Res.
45 (2006) 5708.
6. H. Bernas, K. Eranen, I. Simakova, A.-R. Leino, K. Kordas, J. Myllyoja, P. Maki-Arvela,
T. Salmi, D. Yu. Murzin, Fuel 89 (2010) 2033.
7. J. G. Immer, M. J. Kelly, H. H. Lamb, Appl. Catal. A 375 (2010) 134.
8. P. T. Do, M. Chiappero, L. L. Lobban, D. Resasco, Catal. Lett. 130 (2009) 9.
9. M. Arend, T. Nonnen, W. F. Hoelderich, J. Fischer, J. Groos, Appl. Catal. A, 399 (2011)
198.
10. R. A. Augustine, Heterogeneous Catalysis for the Synthetic Chemist, Marcel Dekker, Inc.,
N. Y., 1996.
11. M. A. Alotaibi, E. F. Kozhevnikova, I. V. Kozhevnikov, J. Catal. 293 (2012) 141.
12. M. A. Alotaibi, E. F. Kozhevnikova, I. V. Kozhevnikov, Chem. Commun. 48 (2012) 7194.
13. S. Itagaki, N. Matsuhashi, K. Taniguchi, K. Yamaguchi, N. Mizuno, Chem. Lett. 43 (2014)
1086.
14. M. A. Alotaibi, E. F. Kozhevnikova, I. V. Kozhevnikov, Appl. Catal. A 447–448 (2012) 32.
15. P. B. Weisz, Adv. Catal. 13 (1962) 137.
16. P. B. Weisz, C. D. Prater, Adv. Catal. 6 (1954) 143.
17. K. Alharbi, E. F. Kozhevnikova, I. V. Kozhevnikov, Appl. Catal. A 504 (2015) 457.
18. G. C. Bond, S. J. Frodsham, P. Jubb, E. F. Kozhevnikova, I. V. Kozhevnikov, J. Catal. 293
(2012) 158.
19. G. C. Bond, Metal-Catalysed Reactions of Hydrocarbons, Springer, N. Y., 2005, Chapter 7.
20. C. K. Ingold, Structure and Mechanism in Organic Chemistry, 2nd ed., Bell, London, 1969.
127
5. Hydrodeoxygenation of 3-pentanone
over bifunctional Pt-heteropoly acid
catalyst in the gas phase: enhancing
effect of gold
5.1 Introduction
As shown in Chapter 4 and reported in other studies [1-5], platinum on acidic supports,
especially, Pt on heteropoly acids (HPA), is a highly active bifunctional metal-acid catalyst for
hydrodeoxygenation (HDO) of a wide range of oxygenates in the gas and liquid phases under
mild conditions. On the other hand, bimetallic PdAu and PtAu catalysts have attracted much
attention because of their enhanced performance in comparison to monometallic Pd and Pt
catalysts [6-17] (and references therein). Bimetallic enhancement of catalyst performance has
been attributed to Au alloying through geometric (ensemble) and electronic (ligand) effects of
the constituent elements [16, 17]. The ensemble effect, often considered to be more important
one [17], can cause structural modifications in the surface metal atom geometry to generate
specific isolated surface sites that are highly active for certain reactions. The ligand effect can
alter the strength of metal-adsorbate bonds as a result of electronic perturbations of platinum
group metal due to heteronuclear metal-metal bond formation, which can also lead to increased
catalyst activity in certain reactions. Bimetallic PdAu catalysts have been extensively
investigated and employed for many important applications, including, among others, the
industrial vinyl acetate synthesis [11, 12], low-temperature CO oxidation [13, 14], and direct
H2O2 synthesis from H2 and O2 [7]. PtAu bimetallics have been widely used for electrocatalysis
in fuel cells [15], but scarcely documented for environment-friendly synthetic applications.
128
The challenge addressed in this chapter is to study the enhancing effect of Au on the gas-phase
HDO of a ketone, 3-pentanone, over bifunctional metal-acid catalysts comprising Pt as the metal
component and a caesium acidic salt of tungstophosphoric HPA, Cs2.5H0.5PW12O40 (CsPW), as
the acid component. This catalyst has been shown to be highly efficient in a wide range of HDO
reactions [1-5]; it has the highest activity in the HDO of anisole [3] and aliphatic ketones [2]
(Chapter 4) for a gas-phase catalyst system reported so far. The HDO of ketones via bifunctional
metal-acid catalysis occurs through a sequence of steps involving hydrogenation of ketone to
secondary alcohol on metal sites followed by dehydration of the alcohol to alkene on acid sites
and finally hydrogenation of the alkene to alkane on metal sites (Scheme 5.1) [1-3]. The
bifunctional metal-acid catalysed pathway has been demonstrated to be much more efficient
compared to the monofunctional metal-catalysed ketone-to-alkane hydrogenation [1, 2] (Chapter
4). It is now demonstrated that modification of the Pt/CsPW catalyst with gold increases the
turnover rate of ketone hydrogenation at Pt surface sites and decreases the rate of catalyst
deactivation. These effects, however, are dependent on the catalyst formulation and preparation
technique. It is suggested that the catalyst enhancement is caused by PtAu alloying. STEM-EDX
and XRD analysis of the PtAu/CsPW catalysts indicates the presence of bimetallic PtAu
nanoparticles with a wide range of Pt/Au atomic ratios.
Scheme 5.1 Ketone hydrodeoxygenation via bifunctional metal-acid catalysis.
Hydrodeoxygenation of 3-pentanone was carried out in the gas phase in flowing H2. The catalysts
were tested under atmospheric pressure in a Pyrex fixed-bed down-flow microreactor (9 mm
internal diameter) fitted with an on-line gas chromatograph (Varian Star 3400 CX instrument
129
with a 30 m x 0.25 mm HP INNOWAX capillary column and a flame ionization detector)
described in Chapter 2. Reaction rates (R) were determined as R = XF/W (in mol gcat-1h-1), where
X is the conversion of 3-pentanone. Turnover frequencies (TOF) were calculated from the
reaction rates using Pt dispersion obtained from hydrogen chemisorption.
5.2 Effect of gold on HDO of 3-pentanone
As demonstrated in Chapter 4, the HDO of aliphatic ketones, including 3-pentanone, over
0.5%Pt/CsPW readily occurs via bifunctional metal-acid catalysed pathway (Scheme 5.1) with
up to 100% alkane yield in a fixed-bed microreactor under mild conditions (60-100 oC, 1 bar H2
pressure). Supported Pt/CsPW and physically mixed Pt/C + CsPW catalysts with the same Pt
loading exhibit comparable activities in the HDO of aliphatic ketones. The alkane/alcohol
product ratio increases with reaction temperature as the result of rate-limiting step change in the
HDO process (Scheme 5.1). 3-Pentanone HDO over 0.5%Pt/CsPW occurs with 80% selectivity
to 3-pentanol at 60 oC and 100% selectivity to pentane at 100 oC (Chapter 4).
In this chapter, we examined the effect of Au additives on activity and performance stability of
physically mixed and supported bifunctional catalysts comprising Pt and CsPW with different
relative amounts of metal and acid components in the HDO of 3-pentanone under kinetically
controlled conditions (<100% ketone conversion) in the temperature range of 40 – 80 oC and
W/F = 400 g h mol-1. The catalyst preparation procedure is explained in detail in Chapter 2, but
briefly summarized here for clarity. The bimetallic PtAu/CsPW catalysts were prepared by two
different methods, co-impregnation and sequential impregnation of CsPW with H2PtCl6 and
HAuCl4 and designated as PtAu/CsPW-CI and PtAu/CsPW-SI respectively. Gold, indeed, was
found to have profound effect on the performance of Pt – CsPW catalysts, subject to catalyst
formulation and preparation method.
130
Addition of Au to Pt/C (Pt/Au = 1:1 and 1:2 atomic ratio) in mixed catalysts Pt/C + CsPW (1:9
w/w, 0.5% Pt loading), did not improve catalyst activity. On the contrary, a decrease in 3-
pentanone conversion was observed regardless of catalyst preparation method, i.e., co-
impregnation or sequential impregnation. Thus, the unmodified 5%Pt/C + CsPW (1:9 w/w)
catalyst gave 70% 3-pentanone conversion with 91% 3-pentanol selectivity at 40 oC, whereas the
Au-modified 5%Pt/5%Au/C + CsPW catalyst with the same Pt loading gave 66% and 82%,
respectively (Table 5.1). Both catalysts showed stable conversion for 4 h on stream. In the
absence of Pt, gold-only catalyst 5%Au/C + CsPW (1:9 w/w) had a negligible activity with only
2% ketone conversion (Table 5.1). Likewise, CsPW alone was totally inert in this reaction at 40-
80 oC (Chapter 4).
131
Table 5.1 3-Pentanone HDO over mixed PtAu/C + CsPW (1:9 w/w) bifunctional catalystsa
a) Reaction conditions: 0.2 g catalyst weight, 40 oC, 1.0% concentration of 3-pentanone in
H2 flow, 20 ml min-1 flow rate, catalyst pre-treatment at 40 oC in H2 for 1 h, 4 h time on
stream.
b) Pt/C, Au/C, and PtAu/C physically mixed with CsPW 1:9 w/w (0.5% Pt loading).
c) Also included cis- and trans-2-pentene at a 5 – 8 pentane/pentene molar ratio.
d) Mainly C1-C4 hydrocarbon cracking products.
e) Catalyst prepared by sequential impregnation by wet-impregnating the pre-made 5%Pt/C
with the required amount of HAuCl4 followed by reduction with H2 at 250 oC/2 h.
f) Catalysts prepared by sequential impregnation by wet-impregnating the pre-made
5%Au/C and 10% Au/C with H2PtCl6.
g) Catalysts prepared by co-impregnation of the Darco KB-B carbon with H2PtCl6 and
HAuCl4 followed by reduction with H2 at 250 oC/2 h.
Next, we looked at a different formulation of the PtAu – CsPW catalysts, with Pt and Au directly
supported on the acidic support CsPW. In these catalysts, Pt and Au sites were in close proximity
to strong proton sites in CsPW also interacting with the ionic surface of CsPW polyoxometalate,
which had profound effect on catalyst performance, i.e., catalyst activity and its resistance to
deactivation.
Catalystb Conversion
(%)
Selectivity (mol%)
Pentanec 3-Pentanol Otherd
5%Pt/C 70 8 91 1
5%Au/C 2
5%Pt/5%Au/C-SIe 66 17 82 1
5%Pt/10%Au/C-SIe 32 20 77 3
5%Pt/5%Au/C-SIf 29 30 67 3
5%Pt/10%Au/C-SIf 25 40 58 2
5%Pt/5%Au/C-CIg 19 46 51 3
5%Pt/10%Au/C-CIg 40 15 83 2
132
Figure 5.1 shows 3-pentanone HDO over unmodified supported catalyst 0.32%Pt/CsPW as well
as the corresponding Au-modified catalysts 0.32%Pt/0.36%Au/CsPW-SI and
0.28%Pt/0.35%Au/CsPW-CI prepared by sequential impregnation and co-impregnation of Pt
and Au, respectively. The results clearly demonstrate enhancement of catalyst performance by
Au additives. First, the co-impregnated catalyst PtAu/CsPW-CI, despite its slightly lower Pt
loading, gives a higher ketone conversion as compared to the unmodified Pt/CsPW, while the
PtAu/CsPW-SI and Pt/CsPW are almost neck and neck. Second, all three catalysts exhibit
deactivation on stream; nevertheless, both Au-modified catalysts deactivate slower than the
unmodified Pt/CsPW. It is conceivable that catalyst deactivation is caused by coking originated
from oligomerization of alkene intermediates (Scheme 5.1) on the strong proton sites of CsPW.
This is supported by propene oligomerization and coking over supported H3PW12O40 [18, 19].
Faster deactivation rate of the supported catalysts in comparison with the mixed ones (PtAu/C +
CsPW) can be explained by the close proximity between the Pt and H+ active sites in the
supported catalysts. Also deactivation of the supported catalysts may be increased due to their
lower Pt loading and higher Pt dispersion (Chapter 3, Table 3.4).
133
Figure 5.1 3-Pentanone HDO over (1) 0.32%Pt/CsPW, (2) 0.32%Pt/0.36%Au/CsPW-SI and (3)
0.28%Pt/0.35%Au/CsPW-CI (0.20 g catalyst weight, 40 oC, ambient pressure, 1.0%
concentration of 3-pentanone in H2 flow, 20 mL min-1 flow rate, catalyst pre-treatment at 40 oC/1
h in H2 flow).
Table 5.2 presents product selectivity for the supported catalysts, which provides an important
insight into the mechanism of Au enhancement. The products contained n-pentane, n-pentenes
(1-pentene, cis- and trans-2-pentene), and 3-pentanol; no products of skeletal isomerization was
observed, which can be explained by low reaction temperature. It should be noted that skeletal
isomerization of n-hexane over Pt/HPA/SiO2 catalysts has been reported at 200 oC [20, 21]. All
three CsPW-supported catalysts, unmodified and Au-modified, had the same 3-pentanol
selectivity of 11%. However, pentane/pentene selectivities differed significantly. The Pt/CsPW
and PtAu/CsPW-SI, that showed similar performance (Figure 5.1), had rather similar selectivities
to pentane (56-62%) and pentenes (27-33%). In contrast, the more active co-impregnated catalyst
PtAu/CsPW-CI gave significantly more pentenes (52%) at the expense of pentane (37%). This
indicates that the Au enhancement of catalyst activity observed for the PtAu/CsPW-CI is largely
3
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134
due to the increased C=O hydrogenation activity, whereas hydrogenation of the alkene C=C
double bond appears to be impeded by the Au additives. It is worth noting that the preference of
Au catalysts to hydrogenation of the C=O bond over C=C bond has been documented previously,
for example, for selective hydrogenation of unsaturated aldehydes to unsaturated alcohols [22,
23], despite the opposite thermodynamic preference [22]. Conversely, Pt alone will preferably
hydrogenate the C=C bond [22].
Table 5.2 HDO of 3-pentanone over Pt/CsPW and PtAu/CsPW.a
a) Reaction conditions: 0.20 g catalyst weight, 40 oC, ambient pressure, 1.0 %
concentration of 3-pentanone in H2 flow, 20 mL min-1 flow rate, catalyst pre-treatment
at 80 oC/1 h in H2 flow, 6 h time on stream.
b) Average 3-pentanone conversion over 6 h time on stream.
c) Catalyst prepared by sequential impregnation of H2PtCl6 then HAuCl4, with Pt(IV)
reduced to Pt(0) with H2 at 250 oC/2 h prior to HAuCl4 impregnation, then the
PtoAuIII/CsPW was reduced with H2 at 250 oC/2 h.
d) Catalysts prepared by co-impregnation of H2PtCl6 and HAuCl4 followed by reduction
with H2 at 250 oC/2 h.
Much more profound effect of Au on catalyst stability was observed in the HDO reaction at 80
oC, i.e., under stronger deactivating conditions (Figure 5.2). Initially, all three CsPW-supported
catalysts exhibited almost 100% 3-pentanone conversion with 100% pentane selectivity. In 6.5
h on stream, the unmodified Pt/CsPW lost 70% of its initial activity and its pentane selectivity
reduced to 95% in favor of 3-pentanol formation. The Au-modified PtAu/CsPW-SI prepared by
sequential impregnation lost half of its activity (49% conversion and 98% pentane selectivity at
Catalyst Conv.b Selectivity (%)
(%) pentane 1-C5H10 trans-2-C5H10 cis-2-C5H10 3-pentanol
0.32%Pt/CsPW 36 56 <1 15 18 11
0.32%Pt/0.36%Au/CsPW-SIc 35 62 <1 13 14 11
0.28%Pt/0.35%Au/CsPW-CId 49 37 <1 23 29 11
135
7 h time on stream). The best performance stability was displayed by the co-impregnated catalyst
PtAu/CsPW-CI, which showed 88% conversion and 100% pentane selectivity after 7 h on
stream. Combustion analysis of spent catalysts indicated that catalyst deactivation rate was in
line with the amount of coke formed, which decreased in the order (C content, %): 2.6 (Pt/CsPW)
> 2.5 (PtAu/CsPW-SI) > 2.2 (PtAu/CsPW-CI).
Figure 5.2 3-Pentanone HDO over (1) 0.32%Pt/CsPW, (2) 0.32%Pt/0.36%Au/CsPW-SI and (3)
0.28%Pt/0.35%Au/CsPW-CI (0.20 g catalyst weight, 80 oC, ambient pressure, 1.0%
concentration of 3-pentanone in H2 flow, 20 mL min-1 flow rate, catalyst pre-treatment at 80 oC/1
h in H2 flow).
Metal dispersion obtained from hydrogen adsorption showed that Pt dispersion in PtAu/CsPW-
CI catalyst was found to be higher in comparison with PtAu/CsPW-SI catalyst (Chapter 3, Table
3.4). This may be the reason for less efficient performance of the catalysts prepared by sequential
impregnation compared with the catalyst prepared by co-impregnation of metal precursors.
3
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Time on stream (min)
136
Further, the effect of Au was tested in a series of bifunctional metal-acid catalysts with various
relative amounts of metal (Pt) and acid (CsPW) components to compare unmodified Pt/CsPW
catalysts with Au-modified co-impregnated PtAu/CsPW-CI catalysts that showed greater
enhancement of catalyst performance.
Figure 5.3 shows the HDO of 3-pentanone at 40 oC over catalysts with reduced acid function.
These catalysts comprised 5.4%Pt/CsPW and 5.3%Pt/3.3%Au/CsPW-CI diluted 1:7 w/w by
SiO2 (0.7% Pt loading). These reactions predictably yielded 3-pentanol as the main product (95-
97% selectivity, Figure 5.4) due to slowing down the alcohol dehydration step (Scheme 5.1). The
unmodified Pt/CsPW catalyst gave 58% average ketone conversion over 4 h on stream, with a
slight catalyst deactivation. The Au-modified catalyst, PtAu/CsPW-CI, again demonstrated
significant enhancement of catalyst activity to exhibit a stable 95% ketone conversion.
Figure 5.3 3-Pentanone HDO over (1) 5.4%Pt/CsPW and (2) 5.3%Pt/3.3%Au/CsPW-CI diluted
1:7 w/w by SiO2 to 0.7% Pt loading (0.20 g catalyst weight, 40 oC, ambient pressure, 1.0% 3-
pentanone concentration in H2 flow, 20 mL min-1 flow rate, catalyst pre-treatment at 40 oC/1 h
in H2 flow).
2
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70
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90
100
0 50 100 150 200 250
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)
Time on stream (min)
137
Figure 5.4 3-Pentanone hydrogenation over (a) 5.4%Pt/CsPW and (b) 5.3%Pt/3.3%Au/CsPW-
CI diluted 1:7 w/w by SiO2 (0.20 g catalyst weight, 40 oC, ambient pressure, 1.0% concentration
of 3-pentanone in H2 flow, 20 mL min-1 flow rate, catalyst pre-treatment at 40 oC/1 h in H2 flow).
Selectivity (a): 3-pentanol, 95%; n-pentane, 3%; other (not shown), 2%; average conversion,
58%; (b): 3-pentanol, 97%; n-pentane, 2%; other (not shown), 1%; average conversion, 95%. In
both cases, cis- and trans-2-pentene, <0.3%.
The reaction with similar catalysts, but with increased acid function, is shown in Figure 5.5. In
this case, 5.8%Pt/CsPW and 5.6%Pt/4.3%Au/CsPW-CI catalysts were diluted 1:19 w/w with
0
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50
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mo
l%)
Time on stream (min)
Conversion
Pentane
3-Pentanol
0
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Conversion
Pentane
3-Pentanol
a)
b)
138
CsPW (0.3% Pt loading). As a result, reaction selectivity changed dramatically to yield mainly
C5 hydrocarbons, i.e., pentane and pentenes (91% selectivity, C5H12/C5H10 = 16 mol/mol for
Pt/CsPW and 86%, C5H12/C5H10 = 6.6 for PtAu/CsPW-CI). The increased catalyst acidity led to
an increase in catalyst deactivation rate (cf. Figure 5.3), and again the Au enhancement of catalyst
stability is clearly visible. Along with reducing the rate of catalyst deactivation, addition of Au
increased the average ketone conversion from 21% for Pt/CsPW to 45% for PtAu/CsPW-CI over
6 h time on stream (Figure 5.5). Again, from the pentane/pentene product ratio, the preference of
PtAu catalyst for the C=O over C=C hydrogenation can be clearly seen, as compared to the
unmodified Pt catalyst (C5H12/C5H10 = 16 and 6.6 mol/mol for Pt/CsPW and PtAu/CsPW-CI,
respectively). It should be noted that the Au-only catalyst, 2.6%Au/CsPW + CsPW (1:19 w/w),
showed only negligible activity (~1% ketone conversion).
Figure 5.5 3-Pentanone HDO over 5.8%Pt/CsPW (solid markers) and 5.6%Pt/4.3%Au/CsPW-
CI (open markers) diluted 1:19 w/w by CsPW to 0.3% Pt loading (0.20 g catalyst weight, 40 oC,
ambient pressure, 1.0% concentration of 3-pentanone in H2 flow, 20 mL min-1 flow rate, catalyst
pre-treatment at 40 oC/1 h in H2 flow).
0
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%)
Time on stream (min)
Conversion C5H12+C5H10 3-Pentanol
Conversion C5H12+C5H10 3-Pentanol
139
Finally, the effect of Au was examined under very strong deactivating conditions at 80 oC using
bifunctional catalysts with greatly increased acid function over metal function. In this case,
5.8%Pt/CsPW and 5.6%Pt/4.3%Au/CsPW-CI were diluted by CsPW 1:79 w/w to 0.07% Pt
loading. The results are shown in Figure 5.6. Initially, in this system pentane was the only product
(~100% selectivity). In the course of reaction, the unmodified Pt/CsPW was severely deactivated
losing practically all its activity in 6 h on stream. Its selectivity was also changed to form 3-
pentanol at the expense of pentane. The Au-modified catalyst was also deactivating, but at a
much slower rate, with its pentane selectivity only slightly changing from 100 to 94%. The
amount of coke formed in the Au-modified catalyst (2.8%) was smaller than in the Pt/CsPW
(3.1%), which is in agreement with the stability of these catalysts.
Figure 5.6 3-Pentanone HDO over 5.8%Pt/CsPW (solid markers) and 5.6%Pt/4.3%Au/CsPW-
CI (open markers) diluted 1:79 w/w by CsPW to 0.07% Pt loading (0.20 g catalyst weight, 80
oC, ambient pressure, 1.0% concentration of 3-pentanone in H2 flow, 20 mL min-1 flow rate,
catalyst pre-treatment at 80 oC/1 h in H2 flow).
0
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Conversion Pentane 3-PentanolConversion Pentane 3-Pentanol
140
Therefore, the modification of Pt/CsPW catalyst with gold increases its activity (ketone
conversion) in the HDO of 3-pentanone and decreases the rate of catalyst deactivation, although
the gold itself is inert in this reaction. The activity enhancement also indicates the preference of
the PtAu/CsPW catalysts toward hydrogenation of C=O bond over C=C bond in comparison with
the unmodified Pt/CsPW. The Au enhancement appears to be strongly dependent on catalyst
formulation as well on the catalyst preparation method. Carbon-supported Pt and Au physically
mixed with CsPW solid acid failed to show any enhancement, whereas the metals directly
supported onto CsPW did display the enhancement effect. This indicates importance of close
proximity between metal and proton active sites in the bifunctional metal-acid catalysts. This
might also indicate a special role of the acidic CsPW polyoxometalate support, however there is
no direct evidence for that as yet. PtAu catalysts prepared by co-impregnation of metal precursors
showed stronger enhancement effect in comparison with the catalysts prepared by sequential
impregnation. It is conceivable that Pt-Au alloying was the cause of the enhancement of catalyst
performance as the result of the ensemble and ligand effects of Au on the Pt active sites [16, 17].
In this respect, the co-impregnation is expected to be more favorable for Pt-Au alloying than the
successive impregnation.
5.3 Catalyst characterisation
This includes investigation of metal nanoparticles in Pt/CsPW and PtAu/CsPW catalysts by X-
ray powder diffraction (XRD) and scanning transmission electron microscopy–energy dispersive
X-ray spectroscopy (STEM-EDX). The STEM-EDX and XRD analysis of the PtAu/CsPW
catalysts indicated the presence of PtAu bimetallic nanoparticles, which may be the cause of
catalyst performance enhancement.
Supported bimetallic catalysts, while preferred for practical use, have a drawback, which is the
lack of homogeneity of metal nanoparticles regarding their composition, size, and shape [17].
141
The method of preparation of the CsPW-supported PtAu catalysts chosen in this work involves
formation of metal nanoparticles at a gas-solid interface upon reduction of a solid pre-catalyst
with H2 at 250 oC. This would favor formation of supported PtAu alloys of a random composition
together with various Pt-alone and Au-alone nanoparticles, rather than specific core-shell
bimetallics often formed in solution in the presence of a protective agent preventing aggregation
[17].
5.3.1 X-ray diffraction
X-ray powder diffraction (XRD) has been widely used for the characterization of supported Au
alloy catalysts [17]. XRD patterns for unmodified 5.8%Pt/CsPW and Au-modified
5.6%Pt/4.3%Au/CsPW-CI catalysts are shown in Figure 5.7a. These are dominated by the well-
known bcc pattern of crystalline CsPW [24] and also clearly display the fcc pattern of Au (38.2o
[111] and 44.4o [200]) and Pt (39.8o [111] and 46.2o [200]) metal nanoparticles. As expected,
this indicates coexistence of Pt-alone and Au-alone particles and possibly PtAu bimetallic
particles with diffraction pattern falling in between the corresponding diffractions of the pure
metals [17]. The latter, however, is obscured by the intense pattern of CsPW in Figure 3.22a.
Nevertheless, the normalized difference XRD (Figure 5.7b) shows a broad diffraction peak in
the range of 38-40o between the diffractions of pure Pt and Au, which could be attributed to PtAu
alloys. It should be noted that Pt peaks appear broader than Au peaks (Figure 5.7a), which
indicates higher dispersion of Pt particles. Accurate analysis of metal particle size is difficult due
to the dominance of the CsPW pattern. Rough estimate from the [111] peaks using the Scherrer
equation gave 60 and 30 nm volume-average particle size for Au and Pt, respectively, which may
be biased toward larger metal particles.
142
Figure 5.7 XRD: (a) 5.8%Pt/CsPW (1) and 5.6%Pt/4.3%Au/CsPW-CI (2); (b) close-up
normalized difference (2)-(1) XRD spectrum revealing a broad [111] fcc PtAu alloy peak in the
range 38-40o and possibly a weaker [200] PtAu alloy peak in the range 44-46o.
5.3.2 STEM-EDX
Scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy
(EDX) have been used extensively for the characterization of PdAu [7, 17, 25] and PtAu [15]
nanoparticles. Figure 5.8 shows the high-angle annular dark field (HAADF) STEM images of
● ●
● Pt
1
a)
○
CsPW+PtAu
●○ ●
○ Au
2
0
1000
2000
3000
4000
5000
6000
10 20 30 40 50 60
Co
un
ts (
cp
s)
2Theta (CuKa) (deg)
b)○
PtAu alloy
○
○ Au
-200
-100
0
100
200
300
400
500
35 37 39 41 43 45 47 49
Co
un
ts (
cp
s)
2Theta (CuKa) (deg)
143
the three catalysts 5.8%Pt/CsPW, 2.6%Au/CsPW and 5.6%Pt/4.3%Au/CsPW-CI with metal
nanoparticles indicated as bright spots on the darker background. As gold, platinum and tungsten
all have similar atomic number Z (74, 78, and 79 for W, Pt, and Au, respectively), the strong
background of CsPW containing 70 wt% of W in these catalysts makes it difficult to discern
smaller Pt and Au particles from the Z-contrast HAADF images. Due to this, no accurate
determination of the metal particle size distribution could be made. Figure 5.8a (sample
5.8%Pt/CsPW) shows platinum particles with a size of ≤ 12 nm. Figure 5.8b (sample
2.6%Au/CsPW) shows oval shaped gold particles sized up between 4 and 25 nm, with an average
gold particle size estimated to be ≤ 10 nm. Particles of a similar size and shape can be seen in
Figure 5.8c (sample 5.6%Pt/4.3%Au/CsPW-CI), which is suggestive of a PtAu alloying on the
catalyst surface (see the EDX analysis below). It can be seen that individual nanoparticles in this
catalyst exhibit well-defined low-index facets with a lattice spacing of 2.3±0.1 Å consistent with
[111] interplanar distances in fcc Au or Pt (Figure 5.8d).
The EDX analysis of a large number of metal nanoparticles in the 5.6%Pt/4.3%Au/CsPW-CI
catalyst showed that all these particles contained both platinum and gold in different Pt/Au
atomic ratios varying from 0.5 to 7.7 (Figure 5.9 and Figure 5.10). This may indicate PtAu
alloying in this catalyst. EDX elemental mapping showed that Pt and Au maps covered the same
areas of PtAu/CsPW catalyst particles (Figure 5.11), indicating formation of a non-uniform PtAu
particles, with local variations in Pt/Au atomic ratio.
144
Figure 5.8 HAADF-STEM images of catalyst samples, showing metal nanoparticles as bright
spots: (a) 5.8%Pt/CsPW, (b) 2.6%Au/CsPW, and (c) 5.6%Pt/4.3%Au/CsPW-CI; a high-
resolution image (d) of sample 5.6%Pt/4.3%Au/CsPW-CI, with fast Fourier transform (FFT) of
the marked area given in the inset, revealing the crystalline structure of the metal nanoparticle.
145
Figure 5.9 (a) HAADF-STEM image of 5.6%Pt/4.3%Au/CsPW-CI catalyst sample, with the
cross on a 12 nm PtAu nanoparticle marking the spot where EDX analysis was performed; (b)
the corresponding EDX spectrum, revealing the atomic ratio Pt/Au = 7.7, indicating that the
probed alloy nanoparticle is Pt-rich.
146
Figure 5.10 STEM-EDX analysis of 5.6%Pt/4.3%Au/CsPW-CI catalyst: (a) HAADF-STEM
image showing two PtAu nanoparticles marked with crosses that were investigated by EDX; (b,
c) the corresponding EDX spectra, revealing the atomic ratio Pt/Au ≈ 0.5 in both spots.
147
Figure 5.11 HAADF-STEM image of 5.6%Pt/4.3%Au/CsPW-CI catalyst and the corresponding
STEM-EDX elemental maps showing the spatial distribution of Au (yellow) and Pt (blue) in the
sample. Note the upper part seem to be relatively Pt rich, whereas the bottom part is Au rich,
indicating non-uniform alloying.
5.4 Turnover rates
The turnover frequencies (TOF) of 3-pentanone conversion over Pt and PtAu catalysts were
calculated from the reaction rate using the Pt dispersion obtained from hydrogen chemisorption
(Chapter 3, Table 3.4). This allowed us to estimate the effect of gold on the intrinsic activity of
Pt surface sites. As the gold alone was practically inactive, the catalyst activity can be attributed
entirely to the Pt sites. Previously, it has been shown that ketone HDO over Pt/CsPW catalyst is
zero order in ketone and near first order in Pt loading [2]. The HDO of 3-pentanone over
5.6%Pt/4.3%Au/CsPW-CI was also found to be zero order in ketone, as the initial reaction rate
practically did not change upon increasing ketone concentration in the feed from 1.0 to 2.0%
(Table 5.3, last two entries). This means that 3-pentanone conversion is equivalent to the reaction
148
rate constant, which allows obtaining TOF values from non-differential conversions. Table 5.3
shows the TOF values thus obtained for Pt/CsPW and PtAu/CsPW catalysts at 40 oC, which were
calculated from the results presented in Figure 5.1 and Figure 5.5 using initial 3-pentanone
conversion (varied between 29 and 60%) and Pt dispersion from Table 3.4 in Chapter 3. These
results show that the turnover rate at Pt sites in the gold-free Pt/CsPW catalysts is weakly
dependent on Pt dispersion, decreasing 2-fold with a 3-fold increase in the Pt dispersion within
the range of 0.32 – 5.8% Pt loading. Gold additives increase the intrinsic activity of Pt surface
sites. More specifically, addition of Au to Pt/CsPW in a Pt/Au molar ratio of about 1:1 and a gold
loading of 0.35 – 4.3% increases the turnover rate at the Pt sites almost 2-fold regardless of the
Pt particle size. This may indicate that Au enhancement of Pt hydrogenation activity is structure
insensitive, as may be expected for catalytic hydrogenation [26].
Table 5.3 Turnover rates for Pt/CsPW and PtAu/CsPW catalysts at 40 oC.a
Catalyst Db
dc
(nm)
Initial
conversion
Initial rate
(mol gcat-1h-1)
TOF
(h-1)
0.32%Pt/CsPW 0.61 1.5 0.490 1.23·10-3 0.63
0.28%Pt/0.35%Au/CsPW-CI 0.55
1.6 0.599 1.50·10-3 0.97
0.32%Pt/0.36%Au/CsPW-SI 0.30
3.0 0.427 1.07·10-3 1.1
5.8%Pt/CsPW 0.19 4.7 0.286 1.43·10-2 1.3
5.6%Pt/4.3%Au/CsPW-CI 0.17 5.3 0.502 2.51·10-2 2.6
5.6%Pt/4.3%Au/CsPW-CId 0.17 5.3 0.225 2.25·10-2 2.3
a) Calculated form the results shown in Figure 5.1 and Figure 5.5 obtained at 1.0%
concentration of 3-pentanone in H2 flow.
b) Pt dispersion (Chapter 3, Table 3.4).
c) Pt particle diameter (Chapter 3, Table 3.4).
d) At 2.0% concentration of 3-pentanone in H2 flow, other conditions as in Figure 5.5.
149
5.5 Conclusions
In this chapter, we have demonstrated the enhancing effect of gold on activity and stability of
Pt/CsPW bifunctional metal-acid catalyst in hydrodeoxygenation (HDO) of 3-pentanone.
Addition of gold to Pt/CsPW has been found to increase both catalyst hydrogenation activity
(turnover rate at Pt sites) and catalyst stability to deactivation, although the Au alone without Pt
is almost totally inert. The bimetallic catalyst PtAu/CsPW shows the preference of C=O over
C=C bond hydrogenation in comparison with the unmodified Pt/CsPW catalyst. The Au
enhancement has been found to be dependent on catalyst formulation as well as catalyst
preparation method. Carbon-supported Pt and Au physically mixed with CsPW solid acid do not
improve catalyst activity. On the other hand, the different formulation of PtAu-CsPW catalyst,
with the metals directly supported on acidic support CsPW, does show the enhancement effect.
PtAu catalyst prepared by sequential Au-after-Pt impregnation shows less enhancement effect in
comparison with the catalysts prepared by co-impregnation.
STEM-EDX and XRD analysis indicates the presence of bimetallic nanoparticles with a wide
range of Pt/Au atomic ratios in the PtAu/CsPW catalysts. The catalyst enhancement can be
attributed to the two previously documented Au alloy effects, i.e., ensemble and ligand effects
[16, 17]. These effects can modify the geometry and electronic state of Pt active sites to enhance
their activity toward C=O bond hydrogenation and reduce catalyst poisoning. Overall, the results
obtained confirm the view that the addition of Au is a promising methodology to enhance the
HDO of biomass-derived feedstock using platinum group metal catalysts [16, 17].
150
5.6 References
1. M. A. Alotaibi, E. F. Kozhevnikova, I. V. Kozhevnikov, J. Catal. 293 (2012) 141.
2. M. A. Alotaibi, E. F. Kozhevnikova, I. V. Kozhevnikov, Chem. Commun. 48 (2012) 7194.
3. K. Alharbi, W. Alharbi, E. F. Kozhevnikova, I. V. Kozhevnikov, ACS Catal. 6 (2016)
2067.
4. S. Itagaki, N. Matsuhashi, K. Taniguchi, K. Yamaguchi, N. Mizuno, Chem. Lett. 43 (2014)
1086.
5. S. Zhu, X. Gao, Y. Zhu, Y. Zhu, X. Xiang, C. Hu, Y. Li, Appl. Catal. B 140-141 (2013) 60.
6. C. Hoang-Van, G. Tournier, S. J. Teichner, J. Catal. 86 (1984) 210.
7. G. J. Hutchings, Chem. Commun. 2008, 1148.
8. K. Q. Sun, Y. C. Hong, G. R. Zhang, B. Q. Xu, ACS Catal. 1 (2011) 1336.
9. D. Tongsakul, S. Nishimura, K. Ebitani, ACS Catal. 3 (2013) 2199.
10. K. Sun, A. R. Wilson, S. T. Thompson, H. H. Lamb, ACS Catal. 5 (2015) 1939.
11. Y. F. Han, J. H. Wang, D. Kumar, Z. Yan, D. W. Goodman, J. Catal. 232 (2005) 467.
12. E. K. Hanrieder, A. Jentys, J. A. Lercher, J. Catal. 333 (2016) 71.
13. J. Xu, T. White, P. Li, C. He, J. Yu, W. Yuan, Y. F. Han, J. Am. Chem. Soc. 132 (2010)
10398.
14. L. B. Ortiz-Soto, O. S. Alexeev, M. D. Amiridis, Langmuir 22 (2006) 3112.
15. R. N. Singh, R. Awasthi, C. S. Sharma, Int. J. Electrochem. Sci. 9 (2014) 5607.
16. B. Coq, F. Figueras, J. Mol. Catal. A 173 (2001) 117.
17. F. Gao, D. W. Goodman, Chem. Soc. Rev. 41 (2012) 8009.
18. M. R. H. Siddiqui, S. Holmes, H. He, W. Smith, E. N. Coker, M. P. Atkins, I. V.
Kozhevnikov, Catal. Lett. 66 (2000) 53.
19. I. V. Kozhevnikov, S. Holmes, M. R. H. Siddiqui, Appl. Catal. A 214 (2001) 47.
20. W. Knaeble, R. T. Carr, E. Iglesia, J. Catal. 319 (2014) 283.
21. T. Pinto, P. Arquilliere, G. P. Niccolai, F. Lefebvre, V. Dufaud, New J.Chem. 39 (2015)
5300.
22. P. Claus, Appl. Catal. A 291 (2005) 222.
23. A. S. K. Hashmi, Chem. Rev. 107 (2007) 3180.
24. T. Okuhara, H. Watanabe, T. Nishimura, K. Inumaru, M. Misono, Chem. Mater. 12 (2000)
2230.
25. K. Sun, A. R. Wilson, S. T. Thompson, H. H. Lamb, ACS Catal. 5 (2015) 1939.
151
26. R. A. van Santen, M. Neurock, in: G. Ertl, H. Knozinger, F. Schuth, J. Weitkamp (Eds.),
Handbook of Heterogeneous Catalysis, 3, Wiley−VCH, 2008, p. 1415.
152
6. Deoxygenation of ethers and esters
over bifunctional Pt-heteropoly acid
catalyst in the gas phase
6.1 Introduction
As stated before by Alotaibi et al. [1, 2] and shown in Chapter 4, platinum on acidic supports,
namely Pt on zeolite HZSM-5 and the acidic caesium salt of tungstophosphoric heteropoly acid
Cs2.5H0.5PW12O40 (CsPW), are highly active bifunctional metal-acid catalysts for hydrogenation
(hydrodeoxygenation) of a wide range of aliphatic and aromatic ketones in the gas phase under
mild conditions to yield the corresponding alkanes.
Here, we investigate the deoxygenation and decomposition of a series of ethers and esters,
including the aromatic ether anisole, the aliphatic diisopropyl ether (DPE) and the aliphatic ester
ethyl propanoate (EP) in the gas phase using bifunctional metal-acid catalysis. The bifunctional
catalysts comprise Pt, Ru, Ni and Cu as the metal components and CsPW as the acid component,
with the main focus on the Pt–CsPW catalyst. It is demonstrated that bifunctional metal-acid
catalysis in the presence of H2 is more efficient for ether and ester deoxygenation than the
corresponding monofunctional metal and acid catalysis. Also it is found that metal- and acid-
catalysed pathways play a different role in these reactions.
Deoxygenation (decomposition) of anisole, DPE and EP ester was carried out in the gas phase in
flowing H2 or N2 under atmospheric pressure in a Pyrex fixed-bed down-flow reactor as
described in Chapter 2.
Information about bifunctional metal-acid and the acid catalysts used in this work is given in
Tables 3.1-3.3 and 3.5 (Chapter 3) including their texture (surface area, pore volume and pore
153
diameter), metal dispersion and acid strength (initial enthalpy of ammonia adsorption). Solid acid
catalysts under study are based on Keggin-type tungsten HPAs, H3PW12O40 and H4SiW12O40,
and possess predominantly Brønsted acid sites. Previously, these catalysts have been thoroughly
characterised using XRD, FTIR, FTIR of adsorbed pyridine, 31P MAS NMR and NH3 adsorption
calorimetry, and their properties have been discussed in detail [3-6]. H2-TPR, XRD and FTIR
studies have shown that CsPW in bifunctional catalysts Pt/CsPW and Pd/CsPW is resistant to
reduction by H2 below 600 oC, and the primary (Keggin) structure of CsPW is retained in CsPW-
supported Pt, Pd and Cu catalysts after treatment with H2 at 400 oC [7], which confirms their
stability under reaction conditions used in this study.
6.2 Hydrogenation of anisole
6.2.1 Catalyst performance
Catalytic hydrogenation of anisole is a model for hydrodeoxygenation of lignin; it has been
extensively studied using both homogeneous and heterogeneous catalysis [8-14] (and references
therein). Representative results for hydrogenation of anisole are given in Table 6.1. Different
catalysts are compared at 100 oC and 1 bar H2 pressure. In anisole conversion over bifunctional
catalysts comprising Pt and CsPW, Pt-catalysed hydrogenation was found to play the key role,
with a relatively moderate assistance of acid catalysis from CsPW. In the absence of Pt, acid-
catalysed conversion of anisole with CsPW was low (19%), yielding mainly phenol (entry 1). In
contrast, Pt-catalysed hydrogenation of anisole, over Pt/C in the absence of CsPW, occurred
readily with 100% conversion and 83% selectivity to cyclohexane, also giving 12% of toluene
by-product (entry 2). The bifunctional Pt-CsPW catalyst comprising a uniform physical mixture
7%Pt/C + CsPW (0.35% Pt content) prepared by grinding a mixture of the two components gave
100% anisole conversion with 98-100% cyclohexane selectivity at 80-100 oC, i.e., almost 100%
cyclohexane yield (entries 3, 4), with stable activity for 20 h on stream as shown in Figure 6.1.
154
This catalyst was highly active even at 60 oC giving 100% anisole conversion with 90%
cyclohexane selectivity (Table 6.1, entry 5). Similar results were obtained when using a mixture
of 10%Pt/SiO2 + CsPW with 0.5% Pt content (entry 6), which exhibited stable performance for
at least 6 h on stream. Methanol was also formed in these reactions with 80-99% selectivity based
on converted anisole (not shown in Table 6.1). The Pt/C + CsPW catalyst with Pt content reduced
to as low as 0.1% showed the same high activity (entry 7). Its activity was dropped only when
the Pt content was further reduced to 0.02% (entries 8, 9, Figure 6.2). Therefore, it is evident that
the Pt metal sites play the primary role in anisole hydrogenation; however, the acid (proton) sites
of CsPW also make significant contribution further enhancing the selectivity to cyclohexane up
to 100%. The Pt/C + CsPW catalyst is more active in comparison with previously reported
catalysts such as Raney Ni and supported Ni, copper-chromite, Mo carbide, Pt/H-Beta, etc.,
which operate in the gas or liquid phase at temperatures of 150-400 oC and elevated H2 pressures
[11-14] (and references therein). Our catalyst compares favorably with the homogeneous
polyoxometalate-stabilized Rh(0) nanocluster, which holds the record lifetime/activity of 2600
TON (turnover numbers) in anisole hydrogenation to methoxycyclohexane (91% yield at 22 oC,
3 bar H2 pressure, 144 h reaction time; reaction in propylene carbonate solution containing
HBF4∙Et2O as an acid promoter) [8]. Our Pt/C + CsPW catalyst is capable of at least 1700 TON
at 100 oC, 1 bar H2 and 20 h on stream (Figure 6.1, Table 6.1), without any problem of catalyst
recovery and reuse.
155
Table 6.1 Hydrogenation of anisole over bifunctional catalysts in the gas phase.a
Entry Catalyst Temp.
(oC)
Conversionb
(%)
Selectivityb (%)
Cyclohexane Otherc
1 CsPW 100 19 (3) 12 88 (60% PhOH)
2 7%Pt/C+SiO2 (1:9 w/w) 100 100 (6) 83 17 (12% PhMe)
3 7%Pt/C+CsPW (1:19 w/w) 100 100 (20) 98 2
4 7%Pt/C+CsPW (1:19 w/w) 80 100 (4) 100 0
5 7%Pt/C+CsPW (1:19 w/w) 60 100 (3) 90 10 (9% MePh)
6 10%Pt/SiO2+CsPW (1:19 w/w) 100 100 (3) 99 1
7 7%Pt/C+CsPW+SiO2 (1:19:60 w/w) 100 100 (6) 98 2
8 7%Pt/C+CsPW+SiO2 (1:19:400 w/w) 100 55 (3) 91 9 (4% PhMe)
9 7%Pt/C+CsPW+SiO2 (1:19:400 w/w) 100 40 (6) 89 11 (4% PhMe)
10 0.5%Pt/CsPW (acac) 100 20 (2) 19 81 (41% PhOH)
11 0.5%Pt/CsPW (H2PtCl6)d 100 87 (2) 89 11 (5% cyclohexanol)
12 5%Ru/CsPWd 100 64 (2) 86 14 (9% PhH)
13 5%Ru/CsPWd 100 23 (4) 85 15 (5% PhH)
14 10%Cu/CsPWd 100 7 (3) 63 37 (8% cyclohexanol)
15 10%Ni/CsPWd 100 10 (3) 22 78 (40% PhOH)
a) Reaction conditions: 0.20 g catalyst weight, 0.50% anisole concentration in H2 flow, 20
ml min-1 flow rate, catalyst pre-treatment at 100 oC for 1 h in H2 flow.
b) Anisole conversion and product selectivity at the time on stream given in round
brackets (in hours); selectivity to methanol (80-99%) not shown.
c) Other: phenol, cyclohexanol, benzene, toluene and unidentified products.
d) Bifunctional metal-acid catalysts were prepared by direct wet impregnation of CsPW
with an appropriate metal precursor (H2PtCl6, RuCl3, Ni(NO3)2, Cu(NO3)2, designated
in Chapters 3 and 4 M/CsPW-I.
156
Figure 6.1 Anisole hydrogenation over bifunctional catalyst 7%Pt/C+CsPW (1:19 w/w, 0.35%
Pt content) (100 oC, 0.20 g catalyst weight, 0.50% anisole concentration in H2 flow, 20 mL
min-1 flow rate, catalyst pre-treatment at 100 oC/1 h in H2 flow. Methanol is not shown; its
selectivity was close to 100%.
Figure 6.2 Anisole hydrogenation over bifunctional catalyst 7%Pt/C+CsPW+SiO2 (1:19:400
w/w) (100 oC, 0.20 g catalyst weight, 0.50% anisole concentration in H2 flow, 20 mL min-1
flow rate, catalyst pre-treatment at 100 oC/1 h in H2 flow (20 mL min-1).
0
10
20
30
40
50
60
70
80
90
100
0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300
Co
nve
rsio
n &
Se
lec
tivit
y (
mo
l %
)
Time (min)
Conversion
Cyclohexane
Cyclohexanol
Others
0
10
20
30
40
50
60
70
80
90
100
0 50 100 150 200 250 300 350
Co
nv
ers
ion
& P
rod
uct
co
mp
osit
ion
(mo
l %
)
Time (min)
Conversion
Cyclohexane
Methanol
Others
157
6.2.2 Effect of catalyst formulation and preparation on the catalyst
performance
It was found that catalyst performance depended greatly on catalyst formulation and preparation.
The Pt/C + CsPW physical mixture was found to be much more active as well as more resistant
to deactivation than the Pt/CsPW catalyst prepared by impregnation of CsPW with a Pt precursor.
Moreover, the type of Pt precursor and the conditions of impregnation were also important for
the performance of catalysts thus made. Thus, 0.5%Pt/CsPW prepared by impregnation of CsPW
with Pt(acac)2 from benzene solution, denoted Pt/CsPW(acac), was less active and less stable to
deactivation than the same catalyst prepared by impregnation with H2PtCl6 from aqueous
solution and denoted Pt/CsPW(H2PtCl6) (cf. Table 6.1, entries 10 and 11). This may be explained
by very different Pt dispersion in these catalysts: 0.46 in Pt/CsPW(acac) and 0.10 in
Pt/CsPW(H2PtCl6) (Table 3.3). It is conceivable that the latter catalyst with larger Pt particles is
more stable to catalyst deactivation, hence its higher catalytic activity. The higher activity and
better performance stability of the Pt/C + CsPW mixture compared with the supported Pt/CsPW
catalyst may be explained assuming that the former catalyst having Pt and proton sites farther
apart suffers less from deactivation (coking) than the latter one with the active sites in close
proximity. This also indicates fast migration of reaction intermediates between metal and acid
sites in the mixed catalyst. Previously, similar behavior, although less pronounced, has been
observed for hydrogenation of acetophenone over Pt/C + CsPW and Pt/CsPW. However, in
hydrogenation of aliphatic ketones these two catalysts showed similar performance (Chapter 4).
Bifunctional catalysts containing other metals supported on CsPW were also tested for
hydrogenation of anisole (Table 6.1, entries 12-15). All these catalysts suffered from catalyst
deactivation. Thus 5%Ru/CsPW gave 64% anisole conversion at 2 h time on stream, which
reduced to 23% at 4 h on stream (entries 12 and 13). Catalyst activity (anisole conversion per
metal weight) was found to decrease in the order of metals: Pt >> Ru > Ni > Cu. The same order
158
of activity has been found for ketone hydrogenation (Chapter 4). Catalyst deactivation can be
attributed to catalyst coking. Table 6.2 shows the amount of carbon in spent catalysts over anisole
hydrogenation at 100 °C.
Table 6.2 C and H combustion analysis for spent M/CsPW catalysts used in the gas phase
hydrogenation of anisole at (100 °C, 3-4 h).
Catalyst C (%) H (%)
0.5%Pt/CsPW (acac) 4.20
0.37
0.5%Pt/CsPW (H2PtCl6) 4.77
0.47
5% Ru/CsPW 4.46
0.51
10%Pt/SiO2 +CsPW
+SiO2(1 / 19 / 400 w/w)
1.55
0.38
CsPW 2.86 0.30
159
6.2.3 Proposed mechanism of anisole hydrogenation over Pt-CsPW
Scheme 6.1 Reaction network for anisole hydrogenation over Pt-CsPW bifunctional catalyst.
In most cases, cyclohexane was the main reaction product (Table 6.1). Among other products
found were methanol, phenol, toluene, benzene and cyclohexanol, in agreement with previous
reports [11-14]. Reaction network for anisole hydrogenation has been discussed elsewhere [8-
14]. It can be represented by Scheme 6.1, which includes metal-catalysed hydrogenation of
aromatic ring (Equation 1), acid-catalysed intra- and intermolecular migration of methyl group
(Equations 2 and 3) and hydrogenolysis of Ar–OH and Ar–OMe bonds on metal sites to yield
benzene and toluene (Equations 4 and 5). The last reaction has been shown to occur readily on
metal complexes and metal nanoclusters [8-10]. Acid-catalysed intra- and intermolecular
migration of methyl group in anisole has been reported previously, for example, over HY zeolite
[15]. As regards the positive effect of acid sites on cyclohexane selectivity in anisole
160
hydrogenation over the Pt/C + CsPW catalyst, it can be attributed to the acid-catalysed
elimination of methanol from methoxylcyclohexane to form cyclohexene followed by its
hydrogenation to cyclohexane (Equation 1).
6.3 Decomposition of diisopropyl ether
6.3.1 Reaction mechanism over Pt-CsPW
Decomposition of the aliphatic diisopropyl ether, DPE, over Pt-CsPW in the presence of
hydrogen can be represented by Equations (6) – (8). These involve the acid-catalysed reactions
of DPE decomposition (6) and isopropanol dehydration (7) to yield propene and the Pt-catalysed
hydrogenation of propene to propane (8).
Scheme 6.2 Decomposition of the aliphatic diisopropyl ether over Pt-CsPW.
With CsPW, possessing strong Brønsted acid sites, reactions (6) and (7) are known to proceed
readily in the gas phase, probably through the mechanism of E1 elimination [3, 4] (Scheme 6.3).
These reactions are reversible and controlled by equilibrium; their Gibbs free energies (∆G) are
14.1 and -0.8 kJ mol-1 for (6) and 7.9 and -4.7 kJ mol-1 for (7) in the gas phase at 25 and 110 oC,
respectively ( the thermodynamic data are given bellow in Table 6.4). In contrast, reaction (8) is
thermodynamically favorable with ∆G = -86.4 kJ mol-1 at 25 oC and -75.3 kJ mol-1 at 110 oC
(Table 6.4). Therefore, it may be expected that with Pt-CsPW bifunctional catalyst under H2 the
decomposition of DPE will be driven forward to form propane as the thermodynamically
favorable product.
161
Scheme 6.3 Mechanism of E1 elimination of DPE and isopropanol.
6.3.2 Thermodynamics of DPE decomposition [16]
Thermodynamic analysis included the flowing reactions:
(iPr)2O = iPrOH + C3H6 (A)
(iPr)2O = 2 C3H6 + H2O (B)
(iPr)2O + 2 H2 = 2 C3H8 + H2O (C)
iPrOH = C3H6 + H2O (D)
C3H6 + H2 = C3H8 (E)
Initial thermodynamic data at standard conditions (Table 6.3) were taken from the literature [17-
21].
Table 6.3 Initial thermodynamic data (298.15 K, 1 bar, ideal gas).
Compound ∆fGo ∆fH
o So Cpo References
kJ mol-1 kJ mol-1 J mol-1K-1 J mol-1K-1
H2 0 0 130.68 28.82 [17]
C3H8 -23.56 -103.85 270.20 73.60 [18]
C3H6 62.84 20.42 266.60 64.31 [19]
H2O -228.6 -241.80 188.8 36.6 [20]
iPrOH -272.6 310.0 89.0 [21]
(iPr)2O -319.0 400a 155a [20]
a) Estimated from values for similar compounds.
162
Table 6.4 Thermodynamic parameters for DPE decomposition at 298.15 and 383.15 K.a
Reaction ∆H298 ∆S298 ∆Cp298 ∆G298 ∆G383 Kp
383 x383
kJ mol-1 J mol-1K-1 J mol-1K-1 kJ mol-1 kJ mol-1
(A) 66.8 176.6 -1.7 14.1 -0.81 1.29 bar 0.75
(B) 118.0 322.0 10.2 22.0 -5.47 5.57 bar2 0.84
(C) -130.5 67.8 -28.8 -150.7 -156.1 1.91 1021 1.00
(D)b 7.9 -4.7
(E)b -86.4 -75.3
a) Undiluted ideal gas system at 1 bar pressure.
b) ∆G values calculated from those for reactions (A) – (D).
Equations (6.1 - 6.7) used for the calculations of thermodynamic parameters for DPE
decomposition are given below, where Kp is the equilibrium constant and x is the equilibrium
conversion of DPE. ∆Cp was assumed to be independent of temperature, i.e., ∆CpT = ∆Cp
298. The
results are presented in Table 6.4 for undiluted ideal gas system. In fact, our reaction system was
diluted with nitrogen, [DPE] = 5.0%. Dilution with inert gas is equivalent to reduction of total
pressure P, which shifts equilibria (A) and (B) towards products. As a result, equilibrium
conversion x will increase. Thus, for reaction (A), x = 0.75 for undiluted system and 0.98 for our
diluted system ([DPE] = 5.0%) at 383.15 K (110 oC). For reaction (B) in diluted system x = 1.
Reaction (C) is volume neutral, i.e., independent of P.
∆HT = ∆H298 + ∆Cp298(T - 298.15) (6.1)
∆ST = ∆S298 + ∆Cp298 ln(T/298.15) (6.2)
∆GT = ∆HT - T∆ST (6.3)
Kp = exp{-∆G/RT} (6.4)
Kp = x2P/(1-x2) for reaction (A), where P is the pressure (6.5)
𝑥 = √𝐾𝑝/(𝑃 + 𝐾𝑝) (6.6)
Kp = 4x3P2/(1-x)(1+2x)2 for reaction (B) (6.7)
163
6.3.3 Decomposition of diisopropyl ether over CsPW and Pt/CsPW
Table 6.5 shows the results for DPE decomposition in the gas phase catalysed by CsPW and Pt-
CsPW under N2 and H2. In the acid-catalysed reaction with CsPW under N2, ether conversion
increased from 6.3 to 96% with increasing the temperature from 50 to 200 oC to give propene
and isopropanol as the products. In this temperature range, propene selectivity increased from 60
to 99% at the expense of isopropanol, indicating the growing contribution of reaction (7) in the
decomposition process as the temperature increased (Figure 6.3). As expected, ether conversion
and product selectivity practically did not change when the CsPW-catalysed reaction was carried
out under H2 instead of N2 (Table 6.5).
Last entries in Table 6.5 show the DPE decomposition in the presence of the bifunctional catalyst
Pt-CsPW under H2 at 110 oC; the catalyst was applied as the uniform 1:19 w/w physical mixture
of 7%Pt/C and CsPW (0.35% Pt content). Pt alone applied as Pt/C + SiO2 under H2 in the absence
of CsPW was inactive in DPE decomposition; nor had the Pt any effect in the reaction with Pt/C
+ CsPW under N2. However, in the reaction with Pt/C + CsPW under H2, the conversion of DPE
greatly increased (from 53 to 99%), giving propane as the main product with 93% selectivity.
Therefore, although DPE decomposed readily without metal assistance via the acid-catalysed
pathway (E1 mechanism) to give propene and isopropanol, the process was greatly accelerated
in the presence of Pt under H2 via the bifunctional metal-acid-catalysed pathway due to shifting
process equilibrium to yield propane, the more thermodynamically favorable product.
164
Table 6.5 Decomposition of diisopropyl ether in the gas phase.a
Catalyst Temperature
(oC)
Conversionb
(%)
Selectivityb (mol %)
Propene Isopropanol Otherc
CsPW 50 6.3 60 40 0
CsPW 70 19 62 38 0
CsPW 110 54 66 33 1
CsPW 110d 56 65e 35 0
CsPW 150 97 98 1 1
CsPW 170 97 98 1 1
CsPW 200 96 99 0 1
7%Pt/C+SiO2f 110d ~1
7%Pt/C+CsPWf 110 53 64 36 0
7%Pt/C+CsPWf 110d 99 93g 2 5h
a) Reaction conditions: 0.20 g catalyst weight, 5.0% diisopropyl ether concentration in N2
flow, 20 ml min-1 flow rate, catalyst pre-treatment at reaction temperature for 1 h in N2
flow (20 mL min-1), 4 h time on stream.
b) Average conversion and product selectivity for 4 h time on stream.
c) Other: acetone, hexene and hexane.
d) Reaction under H2; catalyst pre-treatment at reaction temperature for 1 h in H2 flow.
e) 5% propane + 95% propene.
f) Uniform physical mixture of 7%Pt/C with CsPW or SiO2 (1:19 w/w), 0.35% Pt
content.
g) 100% propane.
h) Hexane.
165
6.3.4 Effect of temperature on DPE decomposition over CsPW
Figure 6.3 shows how the gas phase reaction results vary with increasing temperature (50-200
ºC) for CsPW catalysts. The reactions were carried out for a period of 4 h, at the same conditions
described above. The conversion of DPE increase with increasing in temperature producing
propene and isopropanol. The selectivity toward propene increased with increasing temperature
reached 99% at 150-200 oC.
Figure 6.3 Effect of temperature on diisopropyl ether decomposition catalysed by CsPW (0.20
g catalyst weight, 5.0% diisopropyl ether concentration in N2 flow, 20 mL min-1 flow rate).
0
20
40
60
80
100
0 50 100 150 200
Co
nv
ers
ion
& S
ele
cti
vit
y (
mo
l%)
Temperature (oC)
Conversion
Propene
Isopropanol
166
6.3.5 Catalyst performance stability over CsPW
CsPW catalyst showed no deactivation in extended catalyst stability test for 21 h time on
stream (Figure 6.4).
Figure 6.4 Diisopropyl ether decomposition over CsPW (0.20 g catalyst weight, 110 oC, 5.0%
diisopropyl ether concentration in N2 flow, 20 mL min-1 flow rate, catalyst pre-treatment at 110
oC/1 h in N2 flow).
6.3.6 Kinetic studies
Kinetic studies showed that the CsPW-catalysed reaction was first order in DPE in the DPE
partial pressure range of 1-5 kPa (Figure 6.5); these results were obtained under differential
conditions, i.e., at DPE conversion <10%. The reaction had an apparent activation energy Ea =
50 kJ mol-1 in the temperature range of 50-70 oC. The latter indicates that DPE decomposition
was not limited by mass transport. The absence of pore diffusion limitations was also backed up
by the Weisz-Prater analysis [22] of the reaction system.
0
10
20
30
40
50
60
70
80
90
100
0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300
Co
nv
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ion
& S
ele
cti
vit
y (
mo
l%)
Time (min)
Conversion
Propene
Isopropanol
167
Figure 6.5 Effect of diisopropyl ether partial pressure on the rate of diisopropyl ether
decomposition over CsPW (0.20 g catalyst weight, 50 oC, 20 ml min-1 flow rate).
The activity of a wide range of Keggin-type tungsten HPA catalysts listed in Table 3.2 was tested
in the acid-catalysed decomposition of DPE at 50 oC; the reaction rates and turnover frequencies
(TOF) obtained are given in Table 6.6. TOF is vital measurement used to determine the catalyst
efficiency based on knowledge of catalyst active sites [23].
In this work, the TOF values (h-1) were calculated per surface proton site from the values of DPE
conversion (X) measured under differential conditions (X < 0.1). The required densities of
accessible proton sites were estimated as described elsewhere [3-6]. The protons of our supported
HPA catalysts, which contained HPW or HSiW at sub-monolayer coverage, assumed to be
equally accessible for the reaction (o.16 mmol g-1 for supported 15% HPA and 0.21 mmol g-1 for
15% HSiW). This has been proved in earlier studies by titration of silica-supported HPW with
NH3 [24] and pyridine [25]. For bulk HPW, HSiW and Cs salts of HPW, which have been
0.0000
0.0002
0.0004
0.0006
0.0008
0 1 2 3 4 5 6
Rate
(m
ol g
-1h
-1)
P (kPa)
site activetime
productor reactant of molecules of changeTOF
168
demonstrated to catalyse alcohol dehydration through the surface type mechanism [3-6], the
number of protons on surface was measured using a Keggin unit cross section of 144 Å2 and the
catalyst surface areas from Table 3.2, Chapter 3: HPW (5.6 m2g-1, 0.019 mmol(H+) g-1), HSiW
(9.0 m2g-1. 0.042 mmol(H+) g-1), Cs2.5H0.5PW (132 m2g-1, 0.076 mmol(H+) g-1) and Cs2.25H0.75PW
(128 m2g-1, 0.11 mmol(H+) g-1). The TOF values thus obtained indicate a strong effect of catalyst
acid strength on the turnover reaction rate (see ∆H values in Table 3.5 in Chapter 3).
Table 6.6 Rates of DPE decomposition over HPA catalysts.a
a) Reaction conditions: 50 oC, 0.20 g catalyst weight, 5.0% DPE concentration in N2 flow,
20 mL min-1 flow rate, 4 h time on stream.
b) Average conversion for 4 h time on stream.
c) Rate = XF/W, where X is the conversion, F is the molar flow rate of DPE and W is the
catalyst weight.
d) TOF calculated as the reaction rate per surface proton site.
Figure 6.6 shows a fairly good linear relationship (R2 = 0.871) between the activity of HPA
catalysts in DPE decomposition, ln (TOF), and their acid strength represented by the initial
enthalpy of ammonia adsorption, ΔHNH3 (Table 3.5). Both supported HPA catalysts, bulk Cs salts
of HPW and bulk HPAs (HPW and HSiW) obeyed this plot. This indicates that all these HPA
catalysts operate through the same mechanism of surface catalysis [26, 27] including the bulk
HPAs, for which another, namely a bulk catalysis mechanism, has hitherto been suggested [28]
Catalyst Conversionb 104 Ratec TOFd
mol h-1g-1 h-1
15%HPW/ZrO2 0.0565 7.06 4.53
15%HPW/Nb2O5 0.0384 4.80 3.08
15%HPW/TiO2 0.0539 6.74 4.32
15%HPW/SiO2 0.0672 8.40 5.39
Cs2.5H0.5PW 0.0576 7.20 9.48
Cs2.25H0.75PW 0.0795 9.94 8.95
HSiW 0.0528 6.60 15.9
HPW 0.0479 5.99 30.9
169
(for more discussion on the bulk and surface catalysis mechanisms see [5, 6]). This relationship
can be used to predict the activity of other Brønsted acid catalysts in DPE decomposition from
their ΔHNH3 values and vice versa.
Figure 6.6 Plot of ln (TOF) (TOF in h-1) for diisopropyl ether decomposition versus catalyst acid
strength (50 oC, 0.20 g catalyst weight, 5.0% diisopropyl ether concentration in N2 flow, 20 mL
min-1 flow rate): (1) 15%HPW/ZrO2, (2) 15%HPW/Nb2O5, (3) 15%HPW/TiO2, (4)
15%HPW/SiO2, (5) Cs2.25H0.75PW, (6) Cs2.5H0.5PW, (7) HSiW, (8) HPW.
6.4 Decomposition of ethyl propanoate
6.4.1 Mechanism of acid catalysed decomposition of ethyl propanoate
Mechanistically, the acid-catalysed decomposition of ethyl propanoate, EP, an aliphatic ester,
involves ester protonation to form oxonium ion followed by acyl-oxygen or alkyl-oxygen bond
breaking, which can occur through monomolecular (AAC1 or AAL1) or bimolecular (AAC2 or
AAL2) pathways. This mechanism is well documented for acid-catalysed hydrolysis of esters in
homogeneous solutions [28]. In the gas phase, due to the lack of solvation of cationic
intermediates (acylium and primary alkylcarbenium ions), the acid-catalysed EP decomposition
yielding equimolar mixture of propanoic acid and ethene (see below) is likely to proceed via an
1
2
34
6
5
7
8
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
120 130 140 150 160 170 180 190 200
ln (
TO
F)
-DHNH3 (kJ mol-1)
170
AAL2 mechanism with the alkyl-oxygen bond breaking assisted by a catalyst base site followed
by proton elimination (Scheme 6.4).
Scheme 6.4 AAL2 mechanism of acid-catalysed decomposition of ethyl propanoate (HB is the
catalyst acid site with the conjugate base B).
6.4.2 Decomposition of EP over CsPW and Pt/CsPW
Table 6.7 shows our results for EP decomposition catalysed by CsPW and Pt-CsPW in H2 and
N2 flow. The reaction with CsPW under N2 yielded equimolar mixtures of ethene and
propanoic acid. As expected, EP conversion increased with temperature; at 180 oC, the average
EP conversion was 81% in 4 h on stream.
171
Table 6.7 Decomposition of ethyl propanoate in the gas phase.a
Catalyst
Temperature
(oC)
Conversionb
(%)
Product compositionb,c (mol %)
C2 PA EP Other
CsPW 130d 11 10e 10 79 1
CsPW 180 88 49 45 6 0
CsPW 180d 81 46e 43 11 0
7%Pt/C+SiO2f 180 0.6
7%Pt/C+CsPWf 100 3.0
7%Pt/C+CsPWf 150 31 26 23 50 1
7%Pt/C+CsPWf 170 77g 45 43 12 0
7%Pt/C+CsPWf 180 92 50 46 4 0
7%Pt/C+CsPWf 180d 80 46h 43 11 0
7%Pt/C+CsPWf 200 97 54 45 1 0
a) Reaction conditions: 0.20 g catalyst weight, 0.85% ethyl propanoate concentration in H2
flow, 1 bar pressure, 20 mL min-1 flow rate, catalyst pre-treatment at reaction
temperature for 1 h in H2 flow, 4 h time on stream.
b) Average conversion and product composition for 4 h time on stream.
c) C2 is ethene + ethane, PA is propanoic acid, EP is unconverted ethyl propanoate, other
products are mainly butenes.
d) In N2 flow; catalyst pre-treatment at reaction temperature for 1 h in N2.
e) Ethene only formed.
f) Physical mixture of 7%Pt/C + CsPW or SiO2 (1:19 w/w, 0.35% Pt content).
g) 21 h time on stream.
h) 10% of ethane and 90% of ethene formed.
Table 6.7 also shows the decomposition of EP in the presence of bifunctional catalyst Pt/C +
CsPW under H2; these results were obtained in the temperature range of 100-200 oC. In this
temperature range, the conversion of EP increased from 3 to 97%. The results at 180 oC allow us
to compare EP decomposition through acid-catalysed and metal-acid-catalysed pathways, i.e.,
with CsPW and Pt/C + CsPW catalysts. Pt/C alone (applied as Pt/C + SiO2) in the absence of
CsPW was inactive in EP decomposition (0.6% EP conversion). Addition of Pt/C to CsPW under
172
N2 had practically no effect either; neither conversion nor product selectivity were affected as
compared to the acid-catalysed reaction with CsPW, except for the formation of small amount of
ethane in addition to ethene. Under H2, the Pt/C + CsPW catalyst gave ethane instead of ethene
due to complete hydrogenation of the latter, but EP conversion was practically the same as in the
acid-catalysed reaction with CsPW, indicating no effect of Pt on the reaction rate. This points to
irreversibility of the acid-catalysed decomposition of EP (Scheme 6.4) under reaction conditions
studied.
6.4.3 Catalyst performance stability
At 180 oC, CsPW catalyst suffered from deactivation, with EP conversion decreasing from 88 to
74% in 4 h on stream (Figure 6.5). Initially white, the catalyst turned brown, indicating coke
formation (2.0% of carbon was found in the spent catalyst), which probably caused the observed
catalyst deactivation. The same reaction under H2 gave similar conversion and product
selectivity, and again catalyst deactivation took place (there was 1.2% carbon content in the spent
catalyst).
173
Figure 6.5 Ethyl propanoate decomposition over CsPW (0.20 g catalyst weight, 180 oC, 0.85%
ethyl propanoate concentration in N2 flow, 20 mL min-1 flow rate, catalyst pre-treatment at 180
oC/1 h in N2 flow).
At the same temperature, 180 oC, the Pt/C + CsPW catalyst under H2 had much better
performance stability than CsPW alone; the Pt/C + CsPW catalyst showed no deactivation during
4 h on stream (Figure 6.6, cf. Figure 6.5), with an average EP conversion of 92%, which was
close to the initial EP conversion with CsPW under N2 (88%). Moreover, no deactivation of Pt/C
+ CsPW was observed in extended stability testing for 21 h at 170 oC (Figure 6.7). From these
results it is evident that Pt itself under N2 or H2 is not active in the decomposition of EP in the
temperature range studied, but addition of Pt to CsPW in the presence of H2 improves catalyst
resistance to deactivation. The latter can be explained by reduction of catalyst coking due to
hydrogenation of alkene coke precursors.
0
10
20
30
40
50
60
70
80
90
100
0 50 100 150 200 250 300
Co
nv
ers
ion
& P
rod
uct
co
mp
osit
ion
(m
ol%
)
Time (min)
Conversion
Ethene
Propanoic acid
174
Figure 6.6 Ethyl propanoate hydrogenation over 7%Pt/C+CsPW (1:19 w/w, 0.35% Pt content)
(0.20 g catalyst weight, 180 oC, 0.85% ethyl propanoate concentration in H2 flow, 20 mL min-1
flow rate, catalyst pre-treatment at 180 oC/1 h in H2 flow).
Figure 6.7 Ethyl propanoate decomposition over 7%Pt/C+CsPW (1:19 w/w, 0.35% Pt content)
in flowing H2 (0.20 g catalyst weight, 170 oC, 0.85% ethyl propanoate concentration in H2
flow, 20 mL min-1 flow rate, catalyst pre-treatment at 170 oC/1 h in H2 flow).
0
10
20
30
40
50
60
70
80
90
100
0 50 100 150 200 250
Co
nv
ers
ion
& P
rod
uct
co
mp
osit
ion
(m
ol%
)
Time (min)
Conversion
Ethane
Propanoic acid
0
10
20
30
40
50
60
70
80
90
100
0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300
Co
nv
ers
ion
& P
rod
uct
co
mp
osit
ion
(m
ol%
)
Time (min)
Conversion
Ethane
Propionic acid
175
6.4.4 Kinetic studies
Kinetics of the CsPW-catalysed decomposition of EP was studied under differential conditions
(EP conversion X < 0.1) at 130 oC under N2; at such conditions, no catalyst deactivation was
observed during catalyst testing (4 h time on stream). The reaction was found to be first order in
EP in the ester partial pressure range of 1-8 kPa (Figure 6.8). It obeyed the Arrhenius equation
with apparent activation energy Ea = 72.3 kJ mol-1 in the temperature range of 100-130 oC (Figure
6.9). This Ea value indicates that the reaction occurred without diffusion limitations, as in the
case of DPE decomposition.
Figure 6.8 Effect of ethyl propanoate partial pressure range of 1-8 kPa on the rate of ethyl
propanoate decomposition over CsPW (0.20 g catalyst weight, 130 oC, 20 ml min-1 flow rate).
0.0000
0.0005
0.0010
0.0015
0.0 2.0 4.0 6.0 8.0
Rate
(m
ol g
-1h
-1)
P (kPa)
176
Figure 6.9 Arrhenius plot for ethyl propanoate decomposition over CsPW (X is the conversion
of ethyl propanoate; 0.20 g catalyst weight, 0.85% ethyl propanoate concentration in N2 flow, 20
mL min-1 flow rate, 100-130 oC temperature range; Ea = 72.3 kJ mol-1).
TOF values for the acid-catalysed EP decomposition with the HPA catalysts listed in Table 3.2
are given in Table 6.8. These values were obtained at 130 oC under differential conditions, with
proton site densities determined as in the case of DPE decomposition.
-4.50
-4.00
-3.50
-3.00
-2.50
-2.00
0.00245 0.0025 0.00255 0.0026 0.00265 0.0027
ln X
T-1 (K-1)
177
Table 6.8 Rates of ethyl propanoate decomposition over HPA catalysts.a
Catalyst Conversionb 104 Ratec TOFd
(%) (mol h-1g-1) (h-1)
15%HPW/ZrO2 0.017 0.34 0.22
15%HPW/Nb2O5 0.026 0.51 0.33
15%HPW/TiO2 0.073 1.46 0.94
15%HSiW/SiO2 0.116 2.31 1.11
15%HPW/SiO2 0.101 2.02 1.30
Cs2.5H0.5PW 0.109 2.17 2.86
Cs2.25H0.75PW 0.311 6.22 5.60
HSiW 0.207 4.14 9.97
HPW 0.184 3.67 18.9
a) Reaction conditions: 130 oC, 0.20 g catalyst weight, 0.85% ethyl propanoate
concentration in N2 flow, 20 mL min-1 flow rate, catalyst pre-treatment at 130 oC/1 h in
N2 flow, 4 h time on stream.
b) Average conversion for 4 h time on stream.
c) Rate = XF/W, where X is the conversion, F is the molar flow rate of ethyl propanoate
and W is the catalyst weight.
d) TOF calculated as the reaction rate per surface proton site.
A good activity/acid strength linear correlation (R2 = 0.924) was obtained for the acid-catalysed
EP decomposition (Figure 6.10), as for the decomposition of DPE. This relationship indicates
that all HPA catalysts studied, both bulk and supported, operate through the same mechanism of
surface catalysis.
178
Figure 6.10 Plot of ln (TOF) (TOF in h-1) versus catalyst acid strength (initial enthalpy of NH3
adsorption) for ethyl propanoate decomposition over HPA catalysts (130 oC, 0.20 g catalyst
weight, 0.85% ethyl propanoate concentration in N2 flow, 20 mL min-1 flow rate): (1)
15%HPW/ZrO2, (2) 15%HPW/Nb2O5, (3) 15%HPW/TiO2, (4) 15%HSiW/SiO2, (5)
15%HPW/SiO2, (6) Cs2.25H0.75PW, (7) Cs2.5H0.5PW, (8) HSiW, (9) HPW.
6.5 Conclusion
Here, we have investigated the deoxygenation and decomposition of ethers and esters, including
the aromatic ether anisole, the aliphatic diisopropyl ether (DPE) and the aliphatic ester ethyl
propanoate (EP), using bifunctional metal-acid catalysis at a gas-solid interface in the presence
and absence of hydrogen. The bifunctional catalysts comprise Pt, Ru, Ni and Cu as the metal
components and Cs2.5H0.5PW12O40 (CsPW) as the acid component, with the main focus on the
Pt–CsPW catalyst. It has been demonstrated that bifunctional metal-acid catalysis in the presence
of H2 is more efficient for ether and ester deoxygenation in comparison to the corresponding
monofunctional metal and acid catalysis. Also it has been found that metal- and acid-catalysed
pathways play a different role in these reactions. Hydrodeoxygenation of anisole is a model for
the deoxygenation of lignin; with Pt-CsPW, it occurs with almost 100% yield of cyclohexane
12
3 45
6
7
8
9
-2
-1
0
1
2
3
4
120 130 140 150 160 170 180 190 200
ln (
TO
F)
-ΔHNH3 (kJ mol-1)
179
under very mild conditions at 60-100 oC and 1 bar H2 pressure. In this reaction, Pt-catalysed
hydrogenation plays the key role, with a relatively moderate assistance of acid catalysis, further
increasing the cyclohexane selectivity. The preferred catalyst formulation is a uniform physical
mixture of Pt/C or Pt/SiO2 with excess CsPW, with a Pt content of 0.1-0.5%, which provides
much higher activity and better catalyst stability to deactivation as compared to the Pt/CsPW
catalyst prepared by impregnation of platinum onto CsPW. The Pt/C + CsPW mixed catalyst has
the highest activity in anisole deoxygenation for a gas-phase catalyst system reported so far. On
the other hand, the aliphatic ether DPE decomposes readily over CsPW via acid-catalysed
pathway (E1 mechanism) without metal assistance to give propene and isopropanol, with
propene selectivity increasing with reaction temperature at the expense of isopropanol. Platinum
alone (Pt/C), in the absence of CsPW, is inactive in this reaction, either under H2 or N2. However,
in the presence of Pt-CsPW under H2, DPE decomposition is significantly accelerated, yielding
the more thermodynamically favorable product propane instead of propene. Decomposition of
the EP aliphatic ester is also very efficient via acid-catalysed pathway without metal assistance
to yield ethene and propanoic acid. Addition of Pt to CsPW under H2 causes hydrogenation of
ethene to ethane but does not affect the rate of EP decomposition. Nevertheless, in EP
decomposition, the Pt-CsPW bifunctional catalyst under H2 shows much better performance
stability compared to the CsPW acid catalyst, which can be attributed to reduction of catalyst
coking in the presence of Pt and H2. Kinetics of the acid-catalysed decomposition of DPE and
EP has been studied with a wide range of tungsten HPA catalysts. Good linear relationships
between the logarithm of turnover reaction rate and the HPA catalyst acid strength represented
by ammonia adsorption enthalpies have been demonstrated, which can be used to predict the
activity of other Brønsted acid catalysts in these reactions.
180
6.6 References
1. M. A. Alotaibi, E. F. Kozhevnikova, I. V. Kozhevnikov, J. Catal. 293 (2012) 141.
2. M. A. Alotaibi, E. F. Kozhevnikova, I. V. Kozhevnikov, Chem. Commun. 48 (2012) 7194.
3. A. M. Alsalme, P. V. Wiper, Y. Z. Khimyak, E. F. Kozhevnikova, I. V. Kozhevnikov, J.
Catal. 276 (2010) 181.
4. G. C. Bond, S. J. Frodsham, P. Jubb, E. F. Kozhevnikova, I. V. Kozhevnikov, J. Catal. 293
(2012) 158.
5. W. Alharbi, E. Brown, E. F. Kozhevnikova, I. V. Kozhevnikov, J. Catal. 319 (2014) 174.
6. W. Alharbi, E. F. Kozhevnikova, I. V. Kozhevnikov, ACS Catal. 5 (2015) 7186.
7. M. A. Alotaibi, E. F. Kozhevnikova, I. V. Kozhevnikov. App. Catal. A 447– 448 (2012) 32.
8. J. A. Widegren, R. G. Finke, J. Mol. Catal. A 198 (2003) 198, 317.
9. A. G. Sergeev, J. F. Hartwig, Science 332 (2011) 439.
10. A. G. Sergeev, J. D. Webb, J. F. Hartwig, J. Am. Chem. Soc. 134 (2012) 20226.
11. X. L. Zhu, L. L. Lobban, R. G. Mallinson, D. E. Resasco, J. Catal. 281 (2011) 21.
12. W. Lee, Z. Wang, R. J. Wu, A. Bhan, J. Catal. 2014, 319, 44.
13. Y. Yang, C. Ochoa-Hernandez, V. A. de la Pena O’Shea, P. Pizarro, J. M. Coronado, D. P.
Serrano, Appl. Catal. B 145 (2014) 91.
14. S. Jin, Z. Xiao, C. Li, X. Chen, L. Wang, J. Xing, W. Li, C. Liang, Catal. Today 234 (2014)
125.
15. Q. Meng, H. Fan, H. Liu, H. Zhou, Z. He, Z. Jiang, T. Wu, B. Han, ChemCatChem 7
(2015) 2831.
16. K. Alharbi, W. Alharbi, E. F. Kozhevnikova, I. V. Kozhevnikov, ACS Catal. 6 (2016)
2067.
17. D. D. Wagman, W. H. Evans, V. B. Parker, R. H. Schumm, I. Halow, S. M. Bailey, K. L.
Churney, R. L. J. Nuttall, Phys. Chem. Ref. Data 11 (1982) 1.
18. J. Chao, R. C. Wilhoit, B. J. J. Zwolinski, Phys. Chem. Ref. Data 2 (1973) 427.
19. J. Chao, B. J. J. Zwolinski, Phys. Chem. Ref. Data 4 (1975) 251.
20. CRC Handbook of Chemistry and Physics; Haynes, W. M., ed.; CRC Press, 2012.
21. http://webbook.nist.gov/cgi/cbook.cgi?ID=C67630&Mask=1#Thermo-Gas (accessed
11/01/2016).
22. P. B. Weisz, C. D. Prater, Adv. Catal. 6 (1954) 143.
23. M. Boudart, Chem. Rev. 95 (1995) 661.
24. E. F. Kozhevnikova, I. V. Kozhevnikov, J. Catal. 224 (2004) 164.
181
25. J. Macht, M. Janik, M. Neurock, E. Iglesia, J. Am. Chem. Soc. 130 (2008) 10369.
26. T. Okuhara, N. Mizuno, M. Misono, Adv. Catal. 41 (1996) 113.
27. I. V. Kozhevnikov, Catalysis by Polyoxometalates, Wiley & Sons, Chippenham, 2002, 65.
28. R. A. Y. Jones, Physical and Mechanistic Organic Chemistry; Cambridge University Press:
Cambridge, 1979, 227.
182
7. Conclusion
The aim of this study was to examine the gas phase hydrodeoxygenation (HDO) of a wide range
of oxygenated compounds, such as ketones, ethers and esters, over bifunctional metal acid
catalysis under mild conditions. These catalysts comprise Pt, Ru, Ni and Cu metals supported on
heteropoly acids (HPA), particularly Cs salt of Keggin-type HPA (H3PW12O40),
Cs2.5H0.5PW12O40 (CsPW), which possesses strong proton acidity and high surface area.
Therefore the main focus in our research was on the Pt–CsPW catalyst. Another target of this
study was to investigate the effect of gold additives on activity and performance stability in HDO
of a ketone, 3-pentanone, over Pt-CsPW. A variety of techniques were used to characterise these
catalysts. These include BET, TGA, gas chemisorption, ammonia adsorption calorimetry.
STEM-EDX, XRD, ICP, element analysis (C, H analysis) and FTIR.
In the HDO of ketones we found that:
• The hydrogenation of ketones on supported metal catalysts (e.g. Pt/C and Pd/C) to form
alcohols is feasible, however, further hydrogenation to alkanes is rather difficult to
achieve on such catalysts. The ketone-to-alkane hydrogenation can be achieved much
more easily using bifunctional metal-acid catalysts.
• The bifunctional catalysed HDO of ketones to form alkanes in gas phase occurs via a
sequence of steps involving hydrogenation of ketone to alcohol on metal sites followed
by dehydration of alcohol to alkene on acid sites and finally hydrogenation of alkene to
alkane on metal sites (Scheme 7.1).
• Catalyst activity was found to decrease in the order of metals: Pt > Ru >> Ni > Cu.
• 0.5%Pt/CsPW was demonstrated to be versatile catalyst for the hydrogenation of aliphatic
ketones, giving almost 100% alkane yield at 100 oC and 1 bar pressure.
183
• Evidence was provided that the reaction with Pt/CsPW at 100 oC is limited by ketone-to-
alcohol hydrogenation, whereas at lower temperatures (≤ 60 oC) by alcohol dehydration
resulting in alcohol formation as the main product.
• The catalyst comprising of a physical mixture of 7%Pt/C + CsPW was found to be highly
efficient as well, which indicates that the reaction is not limited by migration of
intermediates between metal and acid sites in the bifunctional catalyst.
• The mixed 7%Pt/C + CsPW catalyst showed better performance stability in acetophenone
hydrogenation (as an aromatic ketone) compared to the impregnated Pt/CsPW catalyst,
which suffered from deactivation.
Scheme 7.1 Ketone hydrogenation via bifunctional metal-acid catalysis.
In the investigation of the effect of gold additives on activity and performance stability in HDO
of ketone, 3-pentanone, over Pt-CsPW, we found that:
• Addition of gold increased the turnover rate of 3-pentanone HDO at Pt sites and decreased
the rate of catalyst deactivation, although the gold itself was inert in this reaction.
• The activity enhancement also indicates the preference of the PtAu/CsPW catalysts
toward hydrogenation of C=O bonds over C=C bonds in comparison with the unmodified
Pt/CsPW.
• The Au enhancement appeared to be strongly dependent on catalyst formulation as well
on the catalyst preparation method:
➢ Carbon-supported Pt and Au physically mixed with CsPW solid acid failed to show
any enhancement, whereas the metals directly supported onto CsPW did display
the enhancement effect. This indicates importance of close proximity between
184
metal and proton active sites in the bifunctional metal-acid catalysts. This might
also indicate a special role of the acidic CsPW polyoxometalate support, however
there is no direct evidence for that as yet.
➢ PtAu catalysts prepared by co-impregnation of metal precursors showed stronger
enhancement effect in comparison with the catalysts prepared by sequential
impregnation.
• STEM-EDX and XRD analysis indicates the presence of bimetallic nanoparticles with a
wide range of Pt/Au atomic ratios in the PtAu/CsPW catalysts.
• The catalyst enhancement can be attributed to the two previously documented Au alloy
effects, i.e., ensemble and ligand effects. These effects can modify the geometry and
electronic state of Pt active sites to enhance their activity toward C=O bond
hydrogenation and reduce catalyst poisoning.
• Overall, the results obtained confirm the view that the addition of Au is a promising
methodology to enhance the HDO of biomass-derived feedstock using platinum group
metal catalysts.
In the HDO of ethers and esters, including the aromatic ether anisole, the aliphatic diisopropyl
ether (DPE) and the aliphatic ester ethyl propanoate (EP), we found that:
• Bifunctional metal-acid catalysis in the presence of H2 was more efficient in comparison
to the corresponding monofunctional metal and acid catalysis. Also we found that metal-
and acid-catalysed pathways play a different role in these reactions.
In the HDO of anisole we found that:
• HDO of anisole with Pt-CsPW occurred with almost 100% yield of cyclohexane under
very mild conditions at 60-100 oC and 1 bar H2 pressure.
185
• In this reaction, Pt-catalysed hydrogenation played the key role, with a relatively
moderate assistance of acid catalysis, further increasing the cyclohexane selectivity.
• The preferred catalyst formulation was a uniform physical mixture of Pt/C or Pt/SiO2
with excess CsPW, with a Pt content of 0.1-0.5%, which provided much higher activity
and better catalyst stability to deactivation as compared to the Pt/CsPW catalyst prepared
by impregnation of platinum onto CsPW.
• The Pt/C + CsPW mixed catalyst showed the highest activity in anisole deoxygenation
for a gas-phase catalyst system reported so far.
In the decomposition of the aliphatic ether DPE:
• DPE decomposed readily over CsPW via acid-catalysed pathway (E1 mechanism)
without metal assistance to give propene and isopropanol, with propene selectivity
increasing with reaction temperature at the expense of isopropanol.
• Platinum alone (Pt/C), in the absence of CsPW, was inactive in this reaction, either under
H2 or N2. However, in the presence of Pt-CsPW under H2, DPE decomposition was
significantly accelerated, yielding the more thermodynamically favorable product
propane instead of propene.
In the decomposition of EP aliphatic ester:
• Decomposition of the EP was very efficient via acid-catalysed pathway without metal
assistance to yield ethene and propanoic acid.
• Addition of Pt to CsPW under H2 caused hydrogenation of ethene to ethane but did not
affect the rate of EP decomposition. Nevertheless, in EP decomposition, the Pt-CsPW
bifunctional catalyst under H2 showed much better performance stability compared to the
CsPW acid catalyst, which can be attributed to reduction of catalyst coking in the
presence of Pt and H2.
186
• Kinetics of the acid-catalysed decomposition of DPE and EP was studied with a wide
range of tungsten HPA catalysts. Good linear relationships between the logarithm of
turnover reaction rate and the HPA catalyst acid strength represented by ammonia
adsorption enthalpies was demonstrated, which can be used to predict the activity of other
Brønsted acid catalysts in these reactions.
Future research on the HDO of organic oxygenates may be aimed at better understanding of
reaction mechanisms. This can be achieved through catalyst characterisation and mechanistic
studies. Another important issue is catalyst deactivation, which hampers the application of HDO.
Therefore, investigation into catalyst deactivation and regeneration could lead to significant
improvement of the HDO methodology.
187
7.1 References
1. M. A. Alotaibi, E. F. Kozhevnikova, I. V. Kozhevnikov, Chem. Commun. 48 (2012)
7194.
2. M. A. Alotaibi, E. F. Kozhevnikova, I. V. Kozhevnikov, J. Catal. 293 (2012) 141.
3. K. Alharbi, E. F. Kozhevnikova, I. V. Kozhevnikov, Appl. Catal. A 504 (2015) 457.
4. O. Poole, K. Alharbi, D. Belic, E. F. Kozhevnikova, I. V. Kozhevnikov, Appl.
Catal. B 202 (2017) 446.
5. K. Alharbi, W. Alharbi, E. F. Kozhevnikova, I. V. Kozhevnikov, ACS Catal. 6
(2016) 2067.
